chore(wecom): make defusedxml dep acquireable and tolerant of absence

Follow-up on top of @TheOnlyMika's #32155 cherry-pick. The defusedxml
hardening import was unconditional, which would break the gateway for
anyone running a WeComCallback adapter without the (transitive-only)
defusedxml present.

- Wrap the import in the same try/except pattern as aiohttp/httpx in
  the same file. Sets DEFUSEDXML_AVAILABLE flag.
- Extend check_wecom_callback_requirements() to gate on the flag, so
  the gateway logs the actual missing dep and skips the adapter
  instead of crashing.
- Add [wecom] extra to pyproject.toml with defusedxml==0.7.1.
- Register platform.wecom_callback in tools/lazy_deps.py so users get
  prompted to install it on first WeComCallback configuration, same
  pattern as discord/slack/matrix.

defusedxml is still the right call for pre-auth XML parsing — this
commit just makes the dep declarative and recoverable instead of a
hard import-time crash.

gateway/run.py

@@ -6294,7 +6294,7 @@ class GatewayRunner:
                 check_wecom_callback_requirements,
             )
             if not check_wecom_callback_requirements():
-                logger.warning("WeComCallback: aiohttp/httpx not installed")
+                logger.warning("WeComCallback: aiohttp/httpx/defusedxml not installed")
                 return None
             return WecomCallbackAdapter(config)