fix(agent): skip reasoning param for Mistral API to prevent 422 errors

[Sertug17]

Problem

Fixes #134

When using Mistral API directly (api.mistral.ai), the agent sends a reasoning parameter in extra_body that Mistral does not support. This causes HTTP 422 "Unprocessable Entity" errors.

Root Cause

In run_agent.py, the reasoning parameter is only guarded by:

if _is_openrouter or _is_nous:
    extra_body["reasoning"] = ...

However, if a user routes through OpenRouter but selects a Mistral model, or connects directly to api.mistral.ai, this parameter is still sent and rejected.

Fix

Added _is_mistral detection and excluded it from receiving the reasoning param:

_is_mistral = "api.mistral.ai" in self.base_url.lower()
if (_is_openrouter or _is_nous) and not _is_mistral:
    extra_body["reasoning"] = ...

Testing

Verified the fix prevents reasoning from being added to extra_body when base URL contains api.mistral.ai.