docs(auth): note getClientRegistrationKind's hydration invariant like getScope

Round-3 @claude review of #1592 (optional nit): getClientRegistrationKind is
the other synchronous getter that, like getScope, is safe only because its
sole caller (buildOAuthConnectionState) awaits getClientInformation/
getServerMetadata first, flushing hydration before this runs on the
post-redirect callback path. Add a docstring mirroring getScope's so a future
refactor doesn't read it without a preceding awaited storage read.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S3fTN8H3R8YV4yUGvZjYnX

core/auth/oauth-storage.ts

@@ -141,6 +141,16 @@ export class OAuthStorageBase implements OAuthStorage {
     return await OAuthClientInformationSchema.parseAsync(clientInfo);
   }
 
+  /**
+   * Intentionally synchronous — same invariant as {@link getScope}: safe
+   * without awaiting hydration only because a preceding awaited read has
+   * already flushed it. Its sole caller, `buildOAuthConnectionState`
+   * (`connection-state.ts`), reads this right after awaiting
+   * `getClientInformation`/`getServerMetadata`, so on the post-redirect
+   * callback path (where the in-memory store is reset) hydration has landed
+   * by the time this runs. A future refactor reading it without a preceding
+   * awaited storage read would get a pre-hydration `undefined`.
+   */
   getClientRegistrationKind(
     serverUrl: string,
   ): OAuthClientRegistrationKind | undefined {