fix(credentials): align EnvVarStorage exists with load semantics (#5680)

* Return boolean from exists method for credential check

* Add test for empty value handling in EnvVarStorage

Add test to verify exists() and load() consistency for empty values in EnvVarStorage.

core/framework/credentials/storage.py

@@ -519,7 +519,7 @@ class EnvVarStorage(CredentialStorage):
     def exists(self, credential_id: str) -> bool:
         """Check if credential is available in environment."""
         env_var = self._get_env_var_name(credential_id)
-        return self._read_env_value(env_var) is not None
+        return bool(self._read_env_value(env_var))
 
     def add_mapping(self, credential_id: str, env_var: str) -> None:
         """

core/framework/credentials/tests/test_credential_store.py

@@ -258,6 +258,14 @@ class TestEnvVarStorage:
         with pytest.raises(NotImplementedError):
             storage.delete("test")
 
+    def test_exists_matches_load_for_empty_value(self):
+        """Test exists() and load() stay consistent for empty values."""
+        storage = EnvVarStorage(env_mapping={"empty": "EMPTY_API_KEY"})
+
+        with patch.object(storage, "_read_env_value", return_value=""):
+            assert storage.load("empty") is None
+            assert not storage.exists("empty")
+
 
 class TestEncryptedFileStorage:
     """Tests for EncryptedFileStorage."""