feat: forward generation params via MITM + add usageMetadata to Gemini
- Add GenerationParams struct to MitmStore for temperature, top_p,
top_k, max_output_tokens, stop_sequences, frequency/presence_penalty
- MITM modify_request injects params into request.generationConfig
- All 3 endpoints (Completions, Responses, Gemini) store client params
- Add usageMetadata to Gemini sync responses (promptTokenCount,
candidatesTokenCount, totalTokenCount, thoughtsTokenCount)
- Add generation param fields to GeminiRequest (temperature, topP, etc.)
- Completions stream_options.include_usage emits final usage chunk
- Completions reasoning_tokens in completion_tokens_details
- Update endpoint gap analysis doc (all high-priority gaps resolved)
b1bd57ab5e