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/Cargo.lock

@@ -44,6 +44,7 @@ version = "0.1.0"
 dependencies = [
  "serde",
  "serde_json",
+ "subtle",
  "wit-bindgen",
 ]
 
@@ -208,6 +209,12 @@ dependencies = [
  "smallvec",
 ]
 
+[[package]]
+name = "subtle"
+version = "2.6.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "13c2bddecc57b384dee18652358fb23172facb8a2c51ccc10d74c157bdea3292"
+
 [[package]]
 name = "syn"
 version = "2.0.117"