oauthex: accept 200 in client registration (#877)

## 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

oauthex/dcr.go

@@ -204,7 +204,7 @@ func RegisterClient(ctx context.Context, registrationEndpoint string, clientMeta
 		return nil, fmt.Errorf("failed to read registration response body: %w", err)
 	}
 
-	if resp.StatusCode == http.StatusCreated {
+	if resp.StatusCode == http.StatusCreated || resp.StatusCode == http.StatusOK {
 		var regResponse ClientRegistrationResponse
 		if err := internaljson.Unmarshal(body, &regResponse); err != nil {
 			return nil, fmt.Errorf("failed to decode successful registration response: %w (%s)", err, string(body))

oauthex/dcr_test.go

@@ -70,6 +70,30 @@ func TestRegisterClient(t *testing.T) {
 			clientMeta:   &ClientRegistrationMetadata{ClientName: "Test App", RedirectURIs: []string{"http://localhost/cb"}},
 			wantClientID: "test-client-id",
 		},
+		{
+			name: "Success with 200",
+			handler: func(w http.ResponseWriter, r *http.Request) {
+				if r.Method != "POST" {
+					t.Errorf("Expected POST, got %s", r.Method)
+				}
+				body, err := io.ReadAll(r.Body)
+				if err != nil {
+					t.Fatal(err)
+				}
+				var receivedMeta ClientRegistrationMetadata
+				if err := json.Unmarshal(body, &receivedMeta); err != nil {
+					t.Fatalf("Failed to unmarshal request body: %v", err)
+				}
+				if receivedMeta.ClientName != "Test App" {
+					t.Errorf("Expected ClientName 'Test App', got '%s'", receivedMeta.ClientName)
+				}
+				w.Header().Set("Content-Type", "application/json")
+				w.WriteHeader(http.StatusOK)
+				w.Write([]byte(`{"client_id":"test-client-id","client_secret":"test-client-secret","client_name":"Test App"}`))
+			},
+			clientMeta:   &ClientRegistrationMetadata{ClientName: "Test App", RedirectURIs: []string{"http://localhost/cb"}},
+			wantClientID: "test-client-id",
+		},
 		{
 			name: "Missing ClientID in Response",
 			handler: func(w http.ResponseWriter, r *http.Request) {