Add response token count logic to Gemini instrumentation.

[umaannamalai]

This PR updates Gemini instrumentation and tests to pull token counts directly from the response object. If a customer has an LLM token counting callback function registered, this will still take priority over the response token count values.

It also removes token count calculations from error cases as the agent will consistently stop reporting token counts when a error was met during the creation of a text generation.

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	6		0	0	0.72s
✅ MARKDOWN	markdownlint	7	0	0	0	1.16s
✅ MARKDOWN	markdown-link-check	7		0	0	24.7s
✅ PYTHON	ruff	940	0	0	0	0.84s
✅ PYTHON	ruff-format	940	1	0	0	0.32s
✅ YAML	prettier	13	0	0	0	1.42s
✅ YAML	v8r	13		0	0	5.83s
✅ YAML	yamllint	13		0	0	0.61s

See detailed report in MegaLinter reports

[codecov-commenter]

Codecov Report

❌ Patch coverage is 67.39130% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.47%. Comparing base (c2f8fe4) to head (68d8f28).
⚠️ Report is 2 commits behind head on feature-llm-token-counts.

Files with missing lines	Patch %	Lines
newrelic/hooks/mlmodel_gemini.py	67.39%	7 Missing and 8 partials ⚠️

Additional details and impacted files

@@                     Coverage Diff                      @@
##           feature-llm-token-counts    #1486      +/-   ##
============================================================
- Coverage                     81.48%   81.47%   -0.01%     
============================================================
  Files                           209      209              
  Lines                         23815    23839      +24     
  Branches                       3753     3757       +4     
============================================================
+ Hits                          19405    19423      +18     
- Misses                         3153     3155       +2     
- Partials                       1257     1261       +4     

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