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/agent/test_redact.py

@@ -1,12 +1,19 @@
 """Tests for agent.redact -- secret masking in logs and output."""
 
 import logging
+import os
 
 import pytest
 
 from agent.redact import redact_sensitive_text, RedactingFormatter
 
 
+@pytest.fixture(autouse=True)
+def _ensure_redaction_enabled(monkeypatch):
+    """Ensure HERMES_REDACT_SECRETS is not disabled by prior test imports."""
+    monkeypatch.delenv("HERMES_REDACT_SECRETS", raising=False)
+
+
 class TestKnownPrefixes:
     def test_openai_sk_key(self):
         text = "Using key sk-proj-abc123def456ghi789jkl012"