## Summary
- Add `ClientCredentialsHandler` implementing `auth.OAuthHandler` using
the OAuth 2.0 Client Credentials grant (RFC 6749 Section 4.4) for
service-to-service authentication with pre-registered credentials.
- Bypasses both dynamic client registration and the authorization code
flow — the handler takes a client ID + secret directly and exchanges
them at the token endpoint.
- Supports two modes: direct `TokenEndpoint` URL, or metadata discovery
via `AuthServerURL` (RFC 8414).
- Add `client_credentials` grant type support to the fake authorization
server in `internal/oauthtest` for testing.
## Context
Per @jba's [comment on
#627](https://github.com/modelcontextprotocol/go-sdk/issues/627#issuecomment-3640982691):
> Our default implementation for that framework should support client ID
and client secret as options, to handle that variant. [...] If those are
set, our implementation would bypass dynamic client reservation. That is
just a couple of lines of code, I believe.
The client-side OAuth scaffolding (#785) that this was blocked on is now
complete, as @brkane noted.
Implements the client credentials variant of
[SEP-1046](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1046).
The JWT Assertions variant (RFC 7523) is left for a follow-up as its API
surface needs more design discussion.
## Test plan
- [x] `TestNewClientCredentialsHandler_Validation` — validates all
config error cases (nil config, missing fields, mutual exclusivity)
- [x] `TestClientCredentialsHandler_Authorize/direct_token_endpoint` —
end-to-end with fake auth server
- [x] `TestClientCredentialsHandler_Authorize/metadata_discovery` —
discovers token endpoint via RFC 8414 metadata
- [x] `TestClientCredentialsHandler_Authorize/bad_credentials` —
verifies failure with wrong secret
- [x] `go test ./... -count=1` passes
- [x] `go vet ./...` clean
Refs #627
Co-authored-by: Maciej Kisiel <mkisiel@google.com>
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