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/sync/session-actions.test.ts

@@ -1014,6 +1014,53 @@ describe("optimisticSend target directory", () => {
     expect(targetStore.getState().part[sentMessageID]?.[0]?.id).toBe("server-part")
   })
 
+  // Relay tunnel aborts carry no HTTP status and no wording the text-matching
+  // heuristic recognizes. Without the transport tag they were classified as
+  // definite failures, the accepted prompt was rolled back, and the queue
+  // re-sent a message the engine was already answering (#2425).
+  test("confirms a tunnel-tagged transport failure that no text heuristic matches", async () => {
+    const targetStore = createStore({})
+    const childStores = createChildStores([["/target/project", targetStore]])
+    let optimisticRemove: OptimisticRemoveCall | null = null
+    let optimisticConfirm: OptimisticRemoveCall | null = null
+    let sentMessageID = ""
+
+    const { markAmbiguousTransportFailure } = await import("@/lib/relay/transport-error")
+    const { optimisticSend, setActionRefs, setOptimisticRefs } = await import("./session-actions")
+    setActionRefs(mockSdk as unknown as OpencodeClient, childStores, () => "/target/project")
+    setOptimisticRefs(
+      () => {},
+      (input) => {
+        optimisticRemove = input
+      },
+      (input) => {
+        optimisticConfirm = input
+      },
+    )
+
+    await optimisticSend({
+      sessionId: "session-tunnel",
+      directory: "/target/project",
+      content: "hello",
+      providerID: "provider",
+      modelID: "model",
+      send: async (messageID) => {
+        sentMessageID = messageID
+        sessionMessagesResult = {
+          data: [{
+            info: { id: messageID, role: "user", sessionID: "session-tunnel", time: { created: 1 } } as Message,
+            parts: [{ id: "server-part", type: "text", text: "hello" } as Part],
+          }],
+        }
+        throw markAmbiguousTransportFailure(new Error("stream aborted by host"))
+      },
+    })
+
+    expect(optimisticRemove).toBe(null)
+    expect((optimisticConfirm as OptimisticRemoveCall | null)?.messageID).toBe(sentMessageID)
+    expect(targetStore.getState().message["session-tunnel"]?.[0]?.id).toBe(sentMessageID)
+  })
+
   test("rolls back an ambiguous send failure when recent messages do not contain the sent ID", async () => {
     const targetStore = createStore({})
     const childStores = createChildStores([["/target/project", targetStore]])