chore: Extended BaseMessage to support reasoning fields and updated usage accounting in streaming mode

[fengju0213]

Description

Describe your changes in detail (optional if the linked issue already contains a detailed description of the changes).

• Reasoning Support: Some providers now emit reasoning_content alongside assistant text (and even during streaming). We plumb that trace directly into BaseMessage so both batch and streaming callers can reliably access the thinking process without scraping metadata.
  • Streaming Usage Handling: Different APIs interleave usage chunks with choice deltas in inconsistent ways. We updated the stream processors to apply usage updates regardless of whether choices are present, ensuring final token accounting and completion emission work across providers like OpenAI, Gemini, and Qwen.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• [ ] Create PR with unit tests
• [ ] Post copyable unit tests in a comment
• [ ] Commit unit tests in branch st_fix_deepseek_stream

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[fengju0213]

updated cc @Wendong-Fan

[fengju0213]

@Wendong-Fan hi，based on our previous disscussion ，we should hanlde thinking content in model level. however ,after further further consideration, i realized that if we want to keep the original response structure，for example， the api returns results that separate thinking and conent，then we should this structure at model level. Once we retain this strcuture at model level, we inevitably need a set of processing logic in the chatagent. therefor , i will continue in the existing manner.

[fengju0213]

LGTM @fengju0213! Could you provide me a model that supports the reasoning_content field for testing purposes?”

@Saedbhati thanks for review!you can use deepseek-reasoner to test it, if you need apikey, you can send me a message in slack

[waleedalzarooni]

Hey @Saedbhati, have you run the example files shall I go ahead and merge

[fengju0213]

Hey @Saedbhati, have you run the example files shall I go ahead and merge

hi @waleedalzarooni the key I gave you should be sufficient to run this example, right? Feel free to message me if you have any questions.

[Saedbhati]

@waleedalzarooni Go head and merge LGTM!