Return 404 for non-existent actions instead of 500
When a POST request is made to a non-existent action endpoint, return a 404 response with NOT_FOUND error code instead of throwing an unhandled ActionNotFoundError.
The fix catches ActionNotFoundError in the action handler and returns an ActionError with code 'NOT_FOUND', which automatically serializes to HTTP 404 status via the built-in codeToStatusMap.
Closes #13549