fix(provider): serialize Mistral prompt cache keys

[rekram1-node]

Summary

• Upgrade @ai-sdk/mistral to 3.0.34
• Patch Mistral provider options to accept promptCacheKey and serialize it as prompt_cache_key
• Add wire-level coverage for the final Mistral request body

Completes the Mistral request behavior introduced in #38424. Split from #35982.

Test plan

• bun test test/provider-mistral.test.ts in packages/core
• bun typecheck in packages/core
• bun typecheck in packages/opencode