Import-time path-template checks now run on both adapters:
- from_jsonschema: validate_path_template moves to forward.rs (same
checks, same messages, shared implementation)
- from_openapi: build_registration calls it before building each op, so
a template like /x{open fails import with 'unterminated placeholder'
instead of a per-call INTERNAL on first invoke
- new test: unterminated_path_template_fails_import_not_first_call