Enhance model handling by normalizing modalities and updating fetch logic; add tests for new functionality

Co-authored-by: Copilot <copilot@github.com>

backend/app/services/openrouter.py

@@ -24,12 +24,25 @@ def _headers() -> dict[str, str]:
     }
 
 
-async def list_models() -> list[dict[str, Any]]:
-    """Return available models from OpenRouter."""
+async def list_models(
+    output_modalities: str = "all",
+    category: str | None = None,
+    supported_parameters: str | None = None,
+) -> list[dict[str, Any]]:
+    """Return available models from OpenRouter.
+
+    Docs: GET /models supports query filters like output_modalities.
+    """
     base_url = os.getenv("OPENROUTER_BASE_URL", OPENROUTER_BASE_URL)
+    params: dict[str, str] = {"output_modalities": output_modalities}
+    if category:
+        params["category"] = category
+    if supported_parameters:
+        params["supported_parameters"] = supported_parameters
+
     async with httpx.AsyncClient(timeout=15) as client:
         resp = client.build_request(
-            "GET", f"{base_url}/models", headers=_headers())
+            "GET", f"{base_url}/models", headers=_headers(), params=params)
         response = await client.send(resp)
         response.raise_for_status()
         return response.json().get("data", [])