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/gateway/test_signal.py

@@ -229,6 +229,10 @@ class TestSignalSessionSource:
 # ---------------------------------------------------------------------------
 
 class TestSignalPhoneRedaction:
+    @pytest.fixture(autouse=True)
+    def _ensure_redaction_enabled(self, monkeypatch):
+        monkeypatch.delenv("HERMES_REDACT_SECRETS", raising=False)
+
     def test_us_number(self):
         from agent.redact import redact_sensitive_text
         result = redact_sensitive_text("Call +15551234567 now")