chore: address review-round-4 nits — stale delenv, upgrade hint, in-sync notes

Four small follow-ups from the reviewer's latest pass:

- tests/unit/test_stdio.py:18: the single_user_env fixture used
  monkeypatch.delenv("ENABLE_MULTI_USER_BASIC_AUTH", ...). That env var
  is no longer read after the ADR-022 follow-up; switched to delenv of
  MCP_DEPLOYMENT_MODE which is the canonical mode-selection input today.
  Comment updated to match.
- config_validators.py: when detect_auth_mode rejects an invalid
  MCP_DEPLOYMENT_MODE, surface a one-line ADR-022 migration hint if the
  rejected value is exactly "oauth_single_audience" (the most common
  upgrade pain — users carrying that value over from ADR-021 .env files).
  Other invalid values get the regular "Valid values: …" message
  unchanged.
- config.py + config_validators.py: added cross-reference comments on
  both mode-resolution sites (Settings.__post_init__ and
  detect_auth_mode) noting that they each compute the canonical mode
  independently and must be kept in sync when a new mode is added.
  Surfaces the parallel-duplication intentionally so the next maintainer
  doesn't have to discover it.
- docs/ADR-021-configuration-consolidation.md:92: appended a trailing
  comment to the historical "valid values" example, marking
  oauth_single_audience and oauth_token_exchange as removed in ADR-022.
  ADR-021 stays as the historical record; the trailer points future
  readers at the current state.

No functional changes; 1009 unit tests still pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/test_stdio.py

@@ -14,8 +14,8 @@ def single_user_env(monkeypatch):
     monkeypatch.setenv("NEXTCLOUD_HOST", "https://cloud.example.com")
     monkeypatch.setenv("NEXTCLOUD_USERNAME", "admin")
     monkeypatch.setenv("NEXTCLOUD_PASSWORD", "secret")
-    # Ensure multi-user mode is off (may leak from other tests)
-    monkeypatch.delenv("ENABLE_MULTI_USER_BASIC_AUTH", raising=False)
+    # Ensure no explicit deployment mode leaks from other tests
+    monkeypatch.delenv("MCP_DEPLOYMENT_MODE", raising=False)
     _reload_config()
     yield
     _reload_config()