fix: preserve OpenRouter Gemini thought signatures

[praneeth999]

Description

Capture and propagate reasoning details(thought_signatures) emitted by OpenRouter Gemini models during streaming responses.

Closes MAS-149
Closes MAS-174

Type of change

• Bug fix (fix:) - Non-breaking change which fixes an issue
• New feature (feat:) - Non-breaking change which adds functionality
• Breaking change (breaking:) - Fix or feature that would cause existing functionality to not work as expected
• Documentation (docs:) - Documentation updates
• Code refactoring (refactor:) - Code changes that neither fix a bug nor add a feature
• Tests (test:) - Adding missing tests or correcting existing tests
• Chore (chore:) - Maintenance tasks, dependency updates, etc.
• Performance improvement (perf:) - Code changes that improve performance
• Code style (style:) - Changes that do not affect the meaning of the code (formatting, missing semi-colons, etc.)
• CI/CD (ci:) - Changes to CI/CD configuration files and scripts

Checklist

• I have run pre-commit on my changed files and all checks pass
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Pre-commit status

# Paste the output of running pre-commit on your changed files:
# uv run pre-commit install
# git diff --name-only HEAD~1 | xargs uv run pre-commit run --files # for last commit
# git diff --name-only origin/<base branch>...HEAD | xargs uv run pre-commit run --files # for all commits in PR
# git add <your file> # if any fixes were applied
# git commit -m "chore: apply pre-commit fixes"
# git push origin <branch-name>

How to Test

Add test method for this PR.

Test CLI Command

Write down the test bash command. If there is pre-requests, please emphasize.

Expected Results

Description/screenshots of expected results.

Additional context

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