fix: require Feishu webhook authentication (#1638)

* fix: require Feishu webhook authentication

* fix: handle Feishu v2 webhook token auth

* fix: skip empty verification token write, consistent with app_id/app_secret

Address zmanian review nit #4: only write verification_token to workspace
when present, matching the if-let pattern used for app_id and app_secret.
Functionally identical (the auth check filters empty strings), but
consistent.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>

channels-src/feishu/feishu.capabilities.json

@@ -27,7 +27,7 @@
       {
         "name": "feishu_verification_token",
         "prompt": "Enter your Feishu/Lark Verification Token (from Event Subscription webhook settings)",
-        "optional": true
+        "optional": false
       }
     ],
     "setup_url": "https://open.feishu.cn/app"
@@ -63,13 +63,15 @@
       },
       "webhook": {
         "secret_header": "X-Feishu-Verification-Token",
-        "secret_name": "feishu_verification_token"
+        "secret_name": "feishu_verification_token",
+        "managed_by_host": false
       }
     }
   },
   "config": {
     "app_id": null,
     "app_secret": null,
+    "verification_token": null,
     "api_base": "https://open.feishu.cn",
     "owner_id": null,
     "dm_policy": "pairing",