fix(ui): surface rejected terminal actions (#124046)

Mark stale terminal tabs exited when the Gateway rejects input or resize, so operators see an actionable outcome instead of silently losing keystrokes.

ui/src/components/terminal/terminal-connection.test.ts

@@ -545,6 +545,29 @@ describe("TerminalConnection", () => {
     ]);
   });
 
+  it.each([
+    ["terminal.input", (conn: TerminalConnection) => conn.input("s1", "echo lost\n")],
+    ["terminal.resize", (conn: TerminalConnection) => conn.resize("s1", 120, 40)],
+  ] as const)("marks the session unavailable when %s rejects its live owner", async (_, act) => {
+    const { client, conn } = makeHarness();
+    const exits: unknown[] = [];
+    await openSession(conn, { onExit: (info) => exits.push(info) });
+    client.nextResponse = { ok: false };
+
+    await act(conn);
+
+    expect(exits).toEqual([
+      {
+        exitCode: null,
+        signal: null,
+        reason: "disconnected",
+        error: "Terminal session is no longer available. Open a new terminal session.",
+      },
+    ]);
+    expect(conn.size).toBe(0);
+    expect(client.listenerCount()).toBe(0);
+  });
+
   it("buffers output that races ahead of sink registration and replays it in order", async () => {
     const { client, conn } = makeHarness();
     const data: string[] = [];

ui/src/components/terminal/terminal-connection.ts

@@ -612,11 +612,25 @@ export class TerminalConnection {
   }
 
   async input(sessionId: string, data: string): Promise<void> {
-    await this.client.request("terminal.input", { sessionId, data }).catch(() => undefined);
+    await this.requestAction("terminal.input", sessionId, { sessionId, data });
   }
 
   async resize(sessionId: string, cols: number, rows: number): Promise<void> {
-    await this.client.request("terminal.resize", { sessionId, cols, rows }).catch(() => undefined);
+    await this.requestAction("terminal.resize", sessionId, { sessionId, cols, rows });
+  }
+
+  private async requestAction(method: string, sessionId: string, params: unknown): Promise<void> {
+    const stream = this.streams.get(sessionId);
+    const result = await this.client.request<{ ok: boolean }>(method, params).catch(() => null);
+    if (result?.ok !== false || !stream || this.streams.get(sessionId) !== stream) {
+      return;
+    }
+    this.deliverExit(sessionId, stream, {
+      exitCode: null,
+      signal: null,
+      reason: "disconnected",
+      error: "Terminal session is no longer available. Open a new terminal session.",
+    });
   }
 
   /** Closes a session server-side and drops its local stream state. */