fix: treat lost relay sends as ambiguous instead of failed

A prompt whose response is lost after the request left the client may
already be running server-side. The relay tunnel reported those failures
as plain text errors ("stream aborted by host", "relay keepalive
timeout"), which matched none of the patterns in isAmbiguousSendFailure,
so an accepted prompt was rolled back and the message queue re-sent it —
two independent AI responses for one user message (#2425). Direct
connections never hit the path.

Transports now tag dispatched-but-unconfirmed failures and the classifier
reads the tag before falling back to status/text heuristics. Confirmation
waits for the connection to actually return (bounded) and retries with
backoff instead of two attempts 150ms apart over the just-broken tunnel.

packages/ui/src/lib/opencode/client.ts

@@ -12,6 +12,7 @@ import type {
   TextPartInput,
   FilePartInput,
 } from "@opencode-ai/sdk/v2";
+import { isAmbiguousTransportFailure, markAmbiguousTransportFailure } from "@/lib/relay/transport-error";
 import type { PermissionRequest } from "@/types/permission";
 import type { QuestionRequest } from "@/types/question";
 
@@ -878,7 +879,13 @@ class OpencodeService {
           // failure) — there is no HTTP response to report. Never fabricate a
           // status: surface it as a transport error so callers treat it like
           // any other network failure instead of a server 500.
-          throw new Error(`Message send transport failure: ${formatSdkError(result.error)}`);
+          // Preserve the transport's "dispatched, outcome unknown" tag through
+          // the wrap: without it the caller cannot tell a lost response from a
+          // send that never reached the server, and re-sends a running prompt.
+          const transportError = new Error(`Message send transport failure: ${formatSdkError(result.error)}`);
+          throw isAmbiguousTransportFailure(result.error)
+            ? markAmbiguousTransportFailure(transportError)
+            : transportError;
         }
         response = new Response(JSON.stringify(result.error), { status });
       } else {