gateway: return actionable error for send channel webchat (openclaw#15703) thanks @rodrigouroz

Verified:
- pnpm build
- pnpm check (fails on current main with unrelated type errors in src/memory/embedding-manager.test-harness.ts)
- pnpm test:macmini (not run after pnpm check failure)
- pnpm test -- src/gateway/server-methods/send.test.ts

Co-authored-by: rodrigouroz <165576107+rodrigouroz@users.noreply.github.com>
Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>

src/gateway/server-methods/send.ts

@@ -106,6 +106,18 @@ export const sendHandlers: GatewayRequestHandlers = {
     const channelInput = typeof request.channel === "string" ? request.channel : undefined;
     const normalizedChannel = channelInput ? normalizeChannelId(channelInput) : null;
     if (channelInput && !normalizedChannel) {
+      const normalizedInput = channelInput.trim().toLowerCase();
+      if (normalizedInput === "webchat") {
+        respond(
+          false,
+          undefined,
+          errorShape(
+            ErrorCodes.INVALID_REQUEST,
+            "unsupported channel: webchat (internal-only). Use `chat.send` for WebChat UI messages or choose a deliverable channel.",
+          ),
+        );
+        return;
+      }
       respond(
         false,
         undefined,