fix(ai): offload model resolution from async paths

Wrap blocking _resolve_model calls in asyncio.to_thread across async model interaction paths so endpoint/model resolution does not stall the event loop. Preserve owner-scoped resolution and add focused regression coverage.

src/agent_tools/session_tools.py

@@ -8,6 +8,7 @@ The session manager is a runtime-set singleton in src.ai_interaction, so each
 function fetches it via get_session_manager() (imported here); _resolve_model and
 AI_CHAT_TIMEOUT are reused from there too.
 """
+import asyncio
 import json
 import logging
 import uuid
@@ -40,7 +41,7 @@ async def create_session(content: str, session_id: Optional[str] = None, owner:
         return {"error": "Session name cannot be empty"}
 
     try:
-        url, model, headers = _resolve_model(model_spec, owner=owner)
+        url, model, headers = await asyncio.to_thread(_resolve_model, model_spec, owner=owner)
     except ValueError as e:
         return {"error": str(e)}