refactor: fold simplify-code review findings

- matrix/dingtalk: extract deps-only installers (ensure_matrix_deps,
  ensure_dingtalk_deps) and register THOSE as ensure_deps_fn — the prior
  check_*_requirements combined credential env checks with the install,
  so a platform configured via PlatformConfig.extra (which is_connected
  accepts) would pass enablement, reach create_adapter(), and have the
  'installer' veto on env-var grounds before installing anything —
  re-creating the #79812 deadlock for extra-configured setups.  The
  combined deps+credentials functions remain for setup/status callers.
- matrix/feishu passive probes: use the existing lazy_deps.is_available()
  instead of hand-rolling 'not feature_missing(...)' (reuse finding).
- teams: module docstring no longer recommends bare system pip (the
  PEP 668 trap purged everywhere else); docs troubleshooting row updated
  to match the new hint text.
- wecom_callback: drop dead 'global ET, DEFUSEDXML_AVAILABLE'
  (ensure_and_bind mutates the module dict directly; nothing assigns).
- tests: parametrized wiring contract for all 8 lazy-installable
  platforms — ensure_deps_fn present and distinct from check_fn
  (behavior contract, not identity snapshot, so renames don't churn it).

plugins/platforms/teams/adapter.py

@@ -6,7 +6,8 @@ Runs an aiohttp webhook server to receive messages from Teams.
 Proactive messaging (send, typing) uses the SDK's App.send() method.
 
 Requires:
-    pip install microsoft-teams-apps aiohttp
+    the ``teams`` extra (auto-installed by the gateway on first start, or
+    manually: ``<hermes-venv>/bin/pip install microsoft-teams-apps aiohttp``)
     TEAMS_CLIENT_ID, TEAMS_CLIENT_SECRET, and TEAMS_TENANT_ID env vars
 
 Configuration in config.yaml: