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/wechat/channel.py

@@ -337,9 +337,21 @@ class WeChatChannel(Channel, WebhookMixin, TokenMixin):
         logger.info(f"WeChat callback POST received, body length={len(body)}")
         xml_data = parse_xml(body)
 
-        # If encrypted, decrypt first
+        # If encryption is configured, the inbound POST MUST carry an
+        # <Encrypt> element and a matching msg_signature. An unsigned body
+        # used to fall through to _safe_process_message and reach the agent
+        # regardless of credentials, which made the encryption setup
+        # ineffective (issue #392). Treat a missing <Encrypt> on an
+        # encryption-configured channel as an authentication failure.
         encrypt = xml_data.get("Encrypt", "")
-        if encrypt and self._crypto:
+        if self._crypto:
+            if not encrypt:
+                logger.warning(
+                    "WeChat POST rejected: encryption is configured but the "
+                    "body has no <Encrypt> element (possible signature bypass)"
+                )
+                return web.Response(status=403)
+
             signature = request.query.get("msg_signature", "")
             timestamp = request.query.get("timestamp", "")
             nonce = request.query.get("nonce", "")