fix(auth): read enumerated issuers exactly, and stop the TUI tone going stale (#2144)

Copilot review round 4.

- `getTokens(url, issuer)` falls back to the legacy unkeyed slot when that
  issuer holds none — right for a connect, wrong for `collectGrants`, which
  labelled the fallback with the issuer it happened to be iterating and paired
  it with that issuer's credentials. During a partially migrated flow the clear
  could have sent an old, unbound token to a newly discovered authorization
  server. `OAuthStorage.getIssuerTokens` is the exact, no-fallback read; only
  the ctx-less read still falls back, which is the one place it belongs.
- The TUI's warning tone was sticky. It lived in its own state while ~30 call
  sites set ordinary OAuth notes without resetting it, so one revocation
  failure left every later "Authorization updated" yellow, across server
  switches. It is now DERIVED — `oauthMessageToneFor(message, warningText)` —
  so it cannot outlive the message that earned it, with nothing to remember to
  clear. Both setters stay plain `useState` setters: a `useCallback` wrapper
  would be a value `react-hooks/exhaustive-deps` demands in seven dependency
  arrays for an identity that never changes.
- Hook-level tests for the web wiring (previously missed): the per-server
  opt-out and the backend-proxied fetch are now asserted to reach
  `clearServerOAuthState`, so handing it the page-origin fetch — which a real
  authorization server's missing CORS headers would reject — fails a test.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0177mPHAdECD18nLLCTwR5rh
Signed-off-by: cliffhall <cliff@futurescale.com>

core/auth/oauth-storage.ts

@@ -424,6 +424,17 @@ export class OAuthStorageBase implements OAuthStorage {
     await this.persist();
   }
 
+  async getIssuerTokens(
+    serverUrl: string,
+    issuer: string,
+  ): Promise<OAuthTokens | undefined> {
+    await this.ensureLoaded();
+    const state = this.memory.getState().getServerState(serverUrl);
+    const tokens = state.byIssuer?.[issuer]?.tokens;
+    if (!tokens) return undefined;
+    return withIssuer(await OAuthTokensSchema.parseAsync(tokens), issuer);
+  }
+
   async listIssuers(serverUrl: string): Promise<string[]> {
     await this.ensureLoaded();
     const state = this.memory.getState().getServerState(serverUrl);