fix(channels): reject unsigned webhook POSTs on encryption-configured channels (#401)

Closes #392 (uncontroversial part).

WeChat (`_handle_message`) and Feishu (`_handle_event`) gated their
signature/decryption checks behind a condition the REQUEST controls:

- WeChat: `if encrypt and self._crypto:` -- a POST with no `<Encrypt>`
  element took the false branch and reached `_safe_process_message`
  without any verification, even when `encoding_aes_key` + `token` were
  configured.
- Feishu: `if self.config.encrypt_key and "encrypt" in body:` -- a
  plaintext body skipped decryption entirely and was processed directly.

Since the webhook port is the channel's only inbound boundary, an
attacker could POST forged plaintext and reach the agent, spoofing
`sender_id` / `FromUserName` (and, with an empty allowlist, passing the
sender gate).

Fix: when encryption is configured, an inbound POST MUST carry the
encrypted field (`<Encrypt>` / `encrypt`) -- otherwise it is rejected
with 403 and never reaches the agent. Plaintext mode (no encryption
configured) is unchanged, so existing plaintext deployments are not
affected. The remaining fail-closed question (what to do when
credentials are entirely unset) is left for the maintainers to decide
as the policy part of the issue.

Regression tests (9 new):
- WeChat: plaintext rejected / missing Encrypt rejected / bad signature
  rejected / valid signature decrypts and processes / plaintext still
  accepted when no crypto.
- Feishu: plaintext rejected / non-dict body rejected / encrypted body
  decrypts and processes / plaintext still accepted when no encrypt_key.

93 tests in the two channel files pass; full suite 3045 passed, 13
skipped; ruff clean.

Co-authored-by: Xi Zhang <106144707+X-iZhang@users.noreply.github.com>

EvoScientist/channels/feishu/channel.py

@@ -828,8 +828,18 @@ class FeishuChannel(Channel, WebhookMixin, TokenMixin):
         except Exception:
             return web.Response(status=400)
 
-        # ── Decrypt if encrypt_key is configured ──
-        if self.config.encrypt_key and "encrypt" in body:
+        # When encryption is configured the inbound POST MUST carry an
+        # ``encrypt`` field. A plaintext body used to skip decryption and
+        # reach the agent directly, defeating the encryption setup (issue
+        # #392). Treat a missing ``encrypt`` field on an
+        # encryption-configured channel as an authentication failure.
+        if self.config.encrypt_key:
+            if not isinstance(body, dict) or "encrypt" not in body:
+                logger.warning(
+                    "Feishu event rejected: encrypt_key is configured but the "
+                    "body has no 'encrypt' field (possible signature bypass)"
+                )
+                return web.Response(status=403)
             try:
                 body = self._decrypt_event(body["encrypt"])
             except Exception: