feat: Add reasoning content for openai model provider

[xiehust]

Description

[Provide a detailed description of the changes in this PR]
Add reasoning content output for OpenAI model provider.
Now agent can output reasoningText when use 'DeepSeek-R1' via OpenAI model provider

async for event in agent.stream_async(query):
      if 'reasoningText' in event:
          print(event['reasoningText'],end="",flush=True)
      if "data" in event:
          # Only stream text chunks to the client
          print(event['data'],end="",flush=True)

Related Issues

[Link to related issues using #issue-number format]

Documentation PR

[Link to related associated PR in the agent-docs repo]

Type of Change

• New feature

Testing

[How have you tested the change?]
YES

• hatch fmt --linter
• hatch fmt --formatter
• hatch test --all
• Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

Checklist

• I have read the CONTRIBUTING document
• I have added tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature
• My changes generate no new warnings
• Any dependent changes have been merged and published

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