fix: strip markdown code blocks from Gemini JSON responses

[aelhajj]

Summary

Strip markdown code block wrappers (```json / ```) from Gemini responses before JSON parsing, fixing a crash when Gemini wraps JSON in markdown despite response_mime_type=application/json being set.

Type of Change

• Bug fix
• New feature
• Performance improvement
• Documentation/Tests

Objective

N/A (bug fix)

Testing

• Unit tests added/updated
• Integration tests added/updated
• All existing tests pass

Tests added in tests/llm_client/test_gemini_client.py covering:

• JSON wrapped in ```json block → parsed correctly
• JSON wrapped in plain ``` block → parsed correctly
• Plain JSON with no wrapper → unchanged, still works

Breaking Changes

• This PR contains breaking changes

Checklist

• Code follows project style guidelines (make lint passes)
• Self-review completed
• Documentation updated where necessary
• No secrets or sensitive information committed

Related Issues

Closes #790

[danielchalef]

All contributors have signed the CLA ✍️ ✅
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[aelhajj]

I have read the CLA Document and I hereby sign the CLA