Shut down asyncgens in routing tests to silence cleanup warnings (PR #5711)

Round 18 reviewers (and earlier) noted CI noise from the routing test
helper: `_drive(coro)` ran a fresh event loop but never closed it or
called `shutdown_asyncgens`, so the MockTransport-backed httpx async
generators in the providers were finalised by GC in a later task and
emitted "Response.aiter_text.aclose was never awaited" / "Task was
destroyed but it is pending" warnings.

Explicitly close the loop after `run_until_complete`, running
`shutdown_asyncgens` first so the iterators finalise in this task.
Tests still pass and the warnings are gone.

studio/backend/tests/test_sampling_params_routing.py

@@ -31,7 +31,18 @@ from core.inference.external_provider import ExternalProviderClient
 
 
 def _drive(coro):
-    return asyncio.new_event_loop().run_until_complete(coro)
+    # Explicit loop lifecycle + asyncgen shutdown so the httpx /
+    # MockTransport-backed async generators in the providers are
+    # finalised in this task instead of being collected later (which
+    # triggers the "aiter_text aclose was never awaited" warning the
+    # reviewer round noticed).
+    loop = asyncio.new_event_loop()
+    try:
+        result = loop.run_until_complete(coro)
+        loop.run_until_complete(loop.shutdown_asyncgens())
+        return result
+    finally:
+        loop.close()
 
 
 def _install_mock(monkeypatch, *, sse_payload: bytes | None = None) -> dict: