## Summary
Accepts HTTP 200 as a valid response in Dynamic Client Registration
(DCR), in addition to 201. Some authorization servers return 200 instead
of the RFC-specified 201 on successful registration.
Fixes https://github.com/modelcontextprotocol/go-sdk/issues/876
## Changes
- `oauthex/dcr.go`: accept 200 status code in client registration
response
- `oauthex/dcr_test.go`: add test coverage for the 200 response case
76d5a54125