Move validateNamespace call outside of filerClient.WithFilerClient closure
so that validation errors return HTTP 400 (InvalidRequest) instead of 500
(InternalError).
Before: Validation error inside closure → treated as internal error → 500
After: Validation error before closure → handled as bad request → 400
This provides correct error semantics: namespace validation is an input
validation issue, not a server error.
08bd1e2563