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.

uv.lock

@@ -1772,6 +1772,9 @@ web = [
     { name = "fastapi" },
     { name = "uvicorn", extra = ["standard"] },
 ]
+wecom = [
+    { name = "defusedxml" },
+]
 youtube = [
     { name = "youtube-transcript-api" },
 ]
@@ -1794,6 +1797,7 @@ requires-dist = [
     { name = "croniter", specifier = "==6.0.0" },
     { name = "daytona", marker = "extra == 'daytona'", specifier = "==0.155.0" },
     { name = "debugpy", marker = "extra == 'dev'", specifier = "==1.8.20" },
+    { name = "defusedxml", marker = "extra == 'wecom'", specifier = "==0.7.1" },
     { name = "dingtalk-stream", marker = "extra == 'dingtalk'", specifier = "==0.24.3" },
     { name = "discord-py", extras = ["voice"], marker = "extra == 'messaging'", specifier = "==2.7.1" },
     { name = "edge-tts", marker = "extra == 'edge-tts'", specifier = "==7.2.7" },
@@ -1876,7 +1880,7 @@ requires-dist = [
     { name = "vercel", marker = "extra == 'vercel'", specifier = "==0.5.7" },
     { name = "youtube-transcript-api", marker = "extra == 'youtube'", specifier = "==1.2.4" },
 ]
-provides-extras = ["anthropic", "exa", "firecrawl", "parallel-web", "fal", "edge-tts", "modal", "daytona", "vercel", "hindsight", "dev", "messaging", "cron", "slack", "matrix", "cli", "tts-premium", "voice", "pty", "honcho", "mcp", "homeassistant", "sms", "computer-use", "acp", "bedrock", "azure-identity", "termux", "termux-all", "dingtalk", "feishu", "google", "youtube", "web", "all"]
+provides-extras = ["anthropic", "exa", "firecrawl", "parallel-web", "fal", "edge-tts", "modal", "daytona", "vercel", "hindsight", "dev", "messaging", "cron", "slack", "matrix", "wecom", "cli", "tts-premium", "voice", "pty", "honcho", "mcp", "homeassistant", "sms", "computer-use", "acp", "bedrock", "azure-identity", "termux", "termux-all", "dingtalk", "feishu", "google", "youtube", "web", "all"]
 
 [[package]]
 name = "hf-xet"