fix: accept logprobs as integer and warn on unsupported params

[Pernekhan]

Summary

• logprobs integer support: Add custom deserializer for chat completion logprobs field to accept both boolean and integer values (0=false, non-zero=true). Some clients send logprobs: 1 instead of logprobs: true.
• Warn instead of reject unsupported params: Change validate_no_unsupported_fields to log a warning and continue instead of returning a 400 error. This improves compatibility with clients that send extra parameters like add_special_tokens, prompt_cache_key, request_id, and chat_template.

Test plan

• New test test_logprobs_accepts_integer verifies integer→bool coercion (0→false, 1→true) and that boolean values still work
• New test test_unsupported_fields_warn_not_error verifies that requests with unknown fields pass validation
• Updated existing tests test_chat_completions_unknown_fields_rejected and test_completions_unsupported_fields_rejected to assert is_ok() instead of is_err()
• All existing async-openai and dynamo-llm tests pass

https://claude.ai/code/session_01EoHsvxn4B3WMuJkwP5f6od