feat(elicitation): add CredentialsNotConfiguredError sentinel

tests/test_setup.py

@@ -0,0 +1,11 @@
+def test_credentials_not_configured_error_exists():
+    from unraid_mcp.core.exceptions import CredentialsNotConfiguredError
+
+    err = CredentialsNotConfiguredError()
+    assert str(err) == "Unraid credentials are not configured."
+
+
+def test_credentials_not_configured_error_is_exception():
+    from unraid_mcp.core.exceptions import CredentialsNotConfiguredError
+
+    assert issubclass(CredentialsNotConfiguredError, Exception)