add chat completion parameter test cases

[knjiang]

Add Chat Completions API parameter test cases to the existing params.ts file for bidirectional mapping coverage.

New test coverage, got the request body from https://platform.openai.com/docs/api-reference/chat/create:

Category	Responses API (existing)	Chat Completions API (new)
Reasoning	effort, summary	+ reasoning_effort
Text config	json_object, json_schema	✓ response_format (both types)
Tools	function, web_search, code_interpreter, specific choice, parallel_tool_calls	✓ function tools, tool_choice, parallel_tool_calls
Context	instructions, truncation	+ system message (maps to instructions)
Storage	store	✓ store
Caching	service_tier, prompt_cache_key	✓ service_tier, prompt_cache_key
Metadata	metadata, safety_identifier	✓ metadata, safety_identifier
Sampling	-	+ temperature, top_p, frequency_penalty, presence_penalty, logprobs, top_logprobs
Output control	-	+ n (multiple completions), stop, max_completion_tokens
Advanced	-	+ prediction, seed, logit_bias

cross compatability coverage report -> https://github.com/braintrustdata/lingua/actions/runs/21009331121

[knjiang]

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• add anthropic messages parameter test cases #59
• add chat completion parameter test cases #58 👈 (View in Graphite)
• add openai responses parameter test cases #54
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