feat(benchmark): Create mock LLM server for use in benchmarks

[tgasser-nv]

• Initial scaffold of mock OpenAI-compatible server
• Refactor mock LLM, fix tests

Description

Related Issue(s)

Checklist

• I've read the CONTRIBUTING guidelines.
• I've updated the documentation if applicable.
• I've added tests if applicable.
• @mentions of the person or team responsible for reviewing proposed changes.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.88%. Comparing base (2af64d6) to head (d9b73be).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1403      +/-   ##
===========================================
+ Coverage    71.66%   71.88%   +0.22%     
===========================================
  Files          171      174       +3     
  Lines        17020    17154     +134     
===========================================
+ Hits         12198    12332     +134     
  Misses        4822     4822              

Flag	Coverage Δ
python	71.88% <100.00%> (+0.22%)	⬆️

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Files with missing lines	Coverage Δ
nemoguardrails/benchmark/mock_llm_server/api.py	100.00% <100.00%> (ø)
nemoguardrails/benchmark/mock_llm_server/models.py	100.00% <100.00%> (ø)
...rdrails/benchmark/mock_llm_server/response_data.py	100.00% <100.00%> (ø)

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