refactor: migrate walker functionality to forge_infra with new abstractions

[tusharmath]

Summary

This refactoring migrates the filesystem walker functionality from forge_services to forge_infra, introducing new infrastructure abstractions that improve separation of concerns and enable better dependency injection patterns. The changes establish a cleaner architecture where infrastructure concerns are properly isolated from service logic.

Key Changes

Infrastructure Migration

• Moved walker implementation from forge_services to forge_infra as
• Added WalkerInfra trait in forge_services/infra.rs for dependency injection
• Created WalkerConfig with conservative and unlimited preset configurations

Service Layer Improvements

• Updated ForgeFsSearch to use WalkerInfra dependency injection instead of direct forge_walker usage
• Enhanced file discovery service to use the new walker infrastructure
• Improved async file operations by replacing File::open with tokio::fs::read_to_string

Testing Enhancements

• Added comprehensive tests for ForgeWalkerService with both conservative and unlimited configurations
• Implemented MockInfra for testing ForgeFsSearch without filesystem dependencies
• Maintained test coverage across all modified components

Technical Impact

• Better separation of concerns: Infrastructure logic now properly separated from service logic
• Improved testability: Services can now be tested with mock infrastructure implementations
• Enhanced maintainability: Clear dependency boundaries make the codebase easier to understand and modify

Files Changed

• : Added walker service and updated dependencies
• : Updated to use new infrastructure abstractions
• files: Updated dependency declarations

This refactoring maintains full backward compatibility while establishing a foundation for future infrastructure improvements.