fix(openai_compat): omit reasoning_content for Mistral requests

[Alix-007]

Problem

• Mistral chat-completions rejects unknown message fields and returns a 422 error ("Extra inputs are not permitted") when reasoning_content is present in history messages.
• openai_compat currently serializes reasoning_content unconditionally, which breaks Mistral compatibility.

Fix

• In openai_compat, sanitize outbound messages for Mistral hosts by omitting reasoning_content before serialization.
• Keep behavior unchanged for other OpenAI-compatible providers.
• Add focused regression tests for Mistral omission and host detection.

Validation

• go test ./pkg/providers/openai_compat -run "TestProviderChat_OmitsReasoningContentForMistral|TestProviderChat_PreservesReasoningContentInHistory|TestIsReasoningContentUnsupportedHost"

Closes #1502