test(agent): make the fake pool's has_available keyword-only like the real one

A positional call would pass the fake but raise TypeError against the real
CredentialPool, which the diagnostic's suppress() would then swallow.

tests/agent/test_init_fallback_refusal_warning.py

@@ -43,7 +43,7 @@ class _Pool:
     def has_credentials(self):
         return True
 
-    def has_available(self, model=None):
+    def has_available(self, *, model=None):
         return self._available