fix(relay): never forward a tunneled body that lost frames (#2822)

When the relay drops mid-request, the prompt_async body frames can be
lost. The tunnel host forwarded the request to loopback as an
empty/truncated chunked body, which the server rejects with a bare 400
(empty response body) — the mobile app's 'Failed to send message (400)'.

Host now buffers request bodies (<512KB) and forwards the complete body
only once StreamEnd arrives; larger bodies still stream live. A new
hasBody flag on the request head lets the host detect a body that
delivered zero frames and abort it as an ambiguous transport failure
(which the client already retries) instead of forwarding an empty body.
A 15s body-delivery deadline converts stalled tunnels into clean aborts.

packages/ui/src/lib/relay/protocol.ts

@@ -61,6 +61,11 @@ export interface TunnelHttpRequestPayload {
   path: string;
   query: string;
   headers: Record<string, string>;
+  /** True when the client had a request body to send. The host uses this to
+   * distinguish a genuine bodyless request from one whose body frames were lost
+   * through the tunnel (which it must abort as an ambiguous transport failure
+   * instead of forwarding an empty body the loopback server rejects with 400). */
+  hasBody?: boolean;
 }
 
 export interface TunnelHttpResponsePayload {

packages/ui/src/lib/relay/tunnel-client.ts

@@ -819,6 +819,7 @@ export const createRelayTunnelClient = (options: RelayTunnelClientOptions): Rela
         path: request.path,
         query: request.query,
         headers: request.headers,
+        hasBody: request.body !== null,
       };
       channel.send(encodeTunnelFrame(TunnelFrameType.HttpRequest, streamId, encodeJsonPayload(head)));
       void (async () => {