fix(tests): resolve all consistently failing tests

- test_plugins.py: remove tests for unimplemented plugin command API
  (get_plugin_command_handler, register_command never existed)
- test_redact.py: add autouse fixture to clear HERMES_REDACT_SECRETS
  env var leaked by cli.py import in other tests
- test_signal.py: same HERMES_REDACT_SECRETS fix for phone redaction
- test_mattermost.py: add @bot_user_id to test messages after the
  mention-only filter was added in #2443
- test_context_token_tracking.py: mock resolve_provider_client for
  openai-codex provider that requires real OAuth credentials

Full suite: 5893 passed, 0 failed.

tests/test_context_token_tracking.py

@@ -30,10 +30,22 @@ class _FakeAnthropicClient:
         pass
 
 
+class _FakeOpenAIClient:
+    """Fake OpenAI client returned by mocked resolve_provider_client."""
+    api_key = "fake-codex-key"
+    base_url = "https://api.openai.com/v1"
+    _default_headers = None
+
+
 def _make_agent(monkeypatch, api_mode, provider, response_fn):
     _patch_bootstrap(monkeypatch)
     if api_mode == "anthropic_messages":
         monkeypatch.setattr("agent.anthropic_adapter.build_anthropic_client", lambda k, b=None: _FakeAnthropicClient())
+    if provider == "openai-codex":
+        monkeypatch.setattr(
+            "agent.auxiliary_client.resolve_provider_client",
+            lambda *a, **kw: (_FakeOpenAIClient(), "test-model"),
+        )
 
     class _A(run_agent.AIAgent):
         def __init__(self, *a, **kw):