fix: remove unnecessary async from root and health endpoints

[xiejianqiao]

## Description                                                                                                                                                             
                                                                                                                                                                           
Remove unnecessary `async` keywords from the `root()` and `health()` endpoints in the FastAPI server. Neither function performs any asynchronous operations, so declaring  them as `async def` adds unnecessary event loop overhead.                                                                                                                    
                                                                                                                                                                           
## Related Issue                                                                                                                                                           
                                                                                                                                                                           
N/A                                                                                                                                                                        
                                                                                                                                                                           
## Type of Change                                                                                                                                                          
                                                                                                                                                                           
- [ ] Bug fix (non-breaking change that fixes an issue)                                                                                                                    
- [ ] New feature (non-breaking change that adds functionality)                                                                                                            
- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)                                                                     
- [ ] Documentation update                                                                                                                                                 
- [ ] Refactoring (no functional changes)                                                                                                                                  
- [x] Performance improvement                                                                                                                                              
- [ ] Test update                                                                                                                                                          
                                                                                                                                                                           
## Changes Made                                                                                                                                                            
                                                                                                                                                                           
- Removed `async` keyword from `root()` endpoint in `server_fastapi.py`                                                                                                    
- Removed `async` keyword from `health()` endpoint in `server_fastapi.py`                                                                                                  
                                                                                                                                                                           
## Testing                                                                                                                                                                 
                                                                                                                                                                           
- [ ] I have added tests that prove my fix is effective or that my feature works                                                                                           
- [ ] New and existing unit tests pass locally with my changes                                                                                                             
- [ ] I have tested this on the following platforms:                                                                                                                       
  - [ ] Linux                                                                                                                                                              
  - [x] macOS                                                                                                                                                              
  - [ ] Windows                                                                                                                                                            
                                                                                                                                                                           
## Checklist                                                                                                                                                               
                                                                                                                                                                           
- [x] My code follows the project's coding style                                                                                                                           
- [x] I have performed a self-review of my code                                                                                                                            
- [ ] I have commented my code, particularly in hard-to-understand areas                                                                                                   
- [ ] I have made corresponding changes to the documentation                                                                                                               
- [x] My changes generate no new warnings                                                                                                                                  
- [x] Any dependent changes have been merged and published                                                                                                                 
                                                                                                                                                                           
## Screenshots (if applicable)                                                                                                                                             
                                                                                                                                                                           
N/A                                                                                                                                                                        
                                                                                                                                                                           
## Additional Notes                                                                                                                                                        
                                                                                                                                                                           
This is a pure refactoring change with no impact on existing functionality or behavior.

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