[FEATURE]: Mistral Provider: Support x-affinity header for session continuation

[machinekoder]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

The official mistral-vibe CLI uses the x-affinity header to track and continue sessions. This seems to be similar to promptCaching in the OpenAI provider for example.
The session_id passed to x-affinity is a UUID4, the session id used or promptCaching in OpenCode is a based on a timestamp, so maybe it's not compatible out of the box, but worth a try.

Here is some evidence:
https://github.com/mistralai/mistral-vibe/blob/9809cfc8315049b6f02b1865306903d92428919f/vibe/core/agent_loop.py#L587
https://github.com/mistralai/mistral-vibe/blob/9809cfc8315049b6f02b1865306903d92428919f/vibe/core/agent_loop.py#L156
https://github.com/mistralai/mistral-vibe/blob/9809cfc8315049b6f02b1865306903d92428919f/vibe/cli/textual_ui/app.py#L1242

[github-actions]

Based on a review of existing issues, I don't see a direct duplicate of this feature request.

However, there are some related issues worth checking:

• /compact doesn't utilize prompt caching #10342: /compact doesn't utilize prompt caching (related to session optimization)
• [FEATURE]: Anthropic (and others) caching improvement #5416: [FEATURE]: Anthropic (and others) caching improvement (similar session continuation patterns across providers)
• [FEATURE]: Header-Based Credential Injection #6088: [FEATURE]: Header-Based Credential Injection (broader custom header support, though focused on credentials rather than session management)

The x-affinity header for Mistral is a specific implementation of session continuation that differs from Anthropic's prompt caching approach. This seems like a new feature request that addresses Mistral's specific session continuation mechanism.