feat: support topk

[KevinZonda]

Describe the change

TopK is well-used paramater for provider which uses OpenAI-compatible API format. This commit add the top_k parameter.

Provide OpenAI documentation link
Provide a relevant API doc from https://platform.openai.com/docs/api-reference

Describe your solution

Add top_k as new parameter

Tests

Additional context

Some evidence that top_k is well used

• https://openrouter.ai/docs/api-reference/parameters
• https://docs.vllm.ai/en/v0.6.4/dev/sampling_params.html

Issue: #641

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.93%. Comparing base (6181fac) to head (203eb5a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #989   +/-   ##
=======================================
  Coverage   85.93%   85.93%           
=======================================
  Files          43       43           
  Lines        2268     2268           
=======================================
  Hits         1949     1949           
  Misses        300      300           
  Partials       19       19           

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