fix: remote engine: handle mistral error on stream mode

[vansangpfiev]

Describe Your Changes

This pull request includes several changes to the engine module, focusing on improving error handling, bypassing model checks, and ensuring proper model path assignment. The most important changes include updates to the StartModel function in models.cc, the StreamWriteCallback function in remote_engine.cc, and the StartModel method in model_service.cc.

Error handling improvements:

• engine/extensions/remote-engine/remote_engine.cc: Updated the StreamWriteCallback function to use Json::Reader for parsing JSON strings, improving error handling for JSON parsing.

Model path assignment:

• engine/controllers/models.cc: Modified the StartModel function to assign the llama_model_path to model_path if model_path is null, ensuring the correct model path is used.

Model check bypass:

• engine/services/model_service.cc: Added a condition to bypass the model check for vision models, allowing the StartModel method to proceed without checking for model existence in the database.

Code formatting:

• engine/services/model_service.cc: Adjusted the formatting of the error message returned when a model fails to start, ensuring better readability.

Fixes Issues

• Closes #
• Closes #

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features)
• Created issues for follow-up changes or refactoring needed