fix: convert reasoning_content to plain string in non‑streaming Gemini

[KaveeshKhattar]

Description

Non-streaming Gemini responses were returning reasoning_content as a nested object instead of a plain string. This breaks OpenAI-compatible clients.

This commit replaces the nested awsbedrock.ReasoningContentBlock with a direct string assignment for the reasoning content.

Related Issues/PRs

Fixes #1974

Validation

• Added TestGeminiCandidatesToOpenAIChoices_ReasoningContent to verify the change.
• make test and make precommit passed locally.

Notes

• Gemini “thought signatures” are not included, as they are not part of the OpenAI schema.

Please suggest if signatures need to be handled differently.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.47%. Comparing base (98200a4) to head (22b701d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1991      +/-   ##
==========================================
+ Coverage   84.33%   84.47%   +0.13%     
==========================================
  Files         130      130              
  Lines       17987    17953      -34     
==========================================
- Hits        15170    15165       -5     
+ Misses       1873     1847      -26     
+ Partials      944      941       -3     

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