Add Gemini test infrastructure.

[umaannamalai]

This PR adds test infrastructure to support embeddings and text completions in google-genai Gemini LLMs including:

• An updated tox file to run tests for google-genai
• A mocked server to return prerecorded Gemini responses to test against so the real Gemini backend with a API key is not needed
• A new conftest.py file that configures and yields test clients, the main server, and includes logic to intercept real Gemini requests and responses and log them out to a log file for later use to update the existing mocked server with new prompts

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	4		0	0	0.52s
✅ MARKDOWN	markdownlint	7	0	0	0	0.99s
✅ MARKDOWN	markdown-link-check	7		0	0	15.92s
✅ PYTHON	ruff	895	0	0	0	0.58s
✅ PYTHON	ruff-format	895	0	0	0	0.32s
✅ YAML	prettier	10	0	0	0	1.03s
✅ YAML	v8r	10		0	0	4.82s
✅ YAML	yamllint	10		0	0	0.52s

See detailed report in MegaLinter reports

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.29%. Comparing base (9223f7a) to head (25705b1).

Additional details and impacted files

@@                        Coverage Diff                         @@
##           feature-gemini-instrumentation    #1343      +/-   ##
==================================================================
- Coverage                           81.48%   81.29%   -0.19%     
==================================================================
  Files                                 204      204              
  Lines                               22821    22821              
  Branches                             3617     3617              
==================================================================
- Hits                                18595    18553      -42     
- Misses                               3013     3050      +37     
- Partials                             1213     1218       +5     

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