fix(tests): add autouse cache-isolation fixture to get_available_models test files

Fixes the CI failure introduced by #817: test_model_resolver::test_custom_endpoint_uses_model_config_api_key_for_model_discovery was failing with KeyError: 'auth' due to the 60s TTL cache in get_available_models() being populated by test_byok_model_dropdown.py tests that ran earlier. Added autouse _isolate_models_cache fixture to 5 test files. Full suite 1747/1747, QA harness green.

tests/test_issue644.py

@@ -3,6 +3,20 @@ import pytest
 import api.config as _cfg
 
 
+@pytest.fixture(autouse=True)
+def _isolate_models_cache():
+    """Invalidate the models TTL cache before and after every test in this file."""
+    try:
+        _cfg.invalidate_models_cache()
+    except Exception:
+        pass
+    yield
+    try:
+        _cfg.invalidate_models_cache()
+    except Exception:
+        pass
+
+
 def _available_models_with_cfg(cfg_override):
     """Helper: temporarily patch config.cfg, call get_available_models(), restore."""
     old_cfg = dict(_cfg.cfg)