Python 3.13 changed str(StrEnum) to return "ClassName.MEMBER" instead of
the plain value. This caused MCP credentials to be stored with provider
"ProviderName.MCP" instead of "mcp", leading to type/provider mismatch
errors during graph validation and execution.
Fix: Pass the enum directly to Pydantic (which extracts .value automatically),
matching the pattern used by all other OAuth handlers. Use .value explicitly
only in non-Pydantic contexts (string comparisons, API calls).