mistral: trace the new Responses API (client.responses) #1003

Sub-issue of #1001 (follow-ups from the 2026-10-26 dependency update, #998).

• mistralai 3.5 added client.responses.create / create_async (typed overloads, stream=True returns an event stream) and count_input_tokens. Nothing in integrations/mistral/patchers.py patches them, so these generation calls go completely untraced.
• Fix: add a patcher and tracing for create / create_async, streaming and non-streaming, gated on mistralai.client.responses existing.
  • Usage carries input_tokens, output_tokens, input_tokens_details.{cached_tokens, cache_write_tokens} and output_tokens_details.reasoning_tokens. These map directly to our prompt_* / completion_* metrics.
  • The shape mirrors OpenAI Responses, so some openai Responses tracing may be reusable.
• Tests: new cassette tests at latest. Older mistral matrix pins skip via a version gate.