fix(web-shell): stop surfacing SSE backpressure warning in transcript (#11340)

The daemon's slow_client_warning frame reports this SSE subscriber's event
queue backing up — a transport diagnostic, not a user-facing failure.
Rendering it as a chat status read as a broken connection mid-turn and sent
users refreshing or abandoning sessions during normal tool execution. Log the
queue watermarks to the console instead; client_evicted remains the real
disconnect signal and keeps its own error path.

Co-authored-by: 钉萁 <[email protected]>

packages/sdk-typescript/src/daemon/ui/normalizer.ts

@@ -190,13 +190,17 @@ export function normalizeDaemonEvent(
         },
       ];
     case 'slow_client_warning':
-      return [
-        {
-          ...base,
-          type: 'status',
-          text: 'SSE stream is lagging',
-        },
-      ];
+      // The daemon reports this SSE subscriber's queue backing up — a
+      // transport diagnostic, not a user-facing failure. Surfacing it as a
+      // transcript status reads as a broken connection mid-turn, so log the
+      // queue watermarks to the console instead; `client_evicted` remains the
+      // real disconnect signal and flows through its own error path. Fires at
+      // most once per overflow episode (daemon-side hysteresis).
+      if (typeof console !== 'undefined') {
+        // eslint-disable-next-line no-console -- intentional diagnostic for subscriber backpressure
+        console.warn?.('[daemon-ui] SSE stream is lagging', event.data);
+      }
+      return [];
     case 'stream_error': {
       const errorKind = asDaemonErrorKind(getString(event.data, 'errorKind'));
       return [

packages/sdk-typescript/test/unit/daemonUi.test.ts

@@ -2436,14 +2436,23 @@ describe('daemon UI normalizer and transcript reducer', () => {
         data: { reason: 'slow' },
       }),
     ).toMatchObject([{ type: 'error', recoverable: true, text: 'slow' }]);
-    expect(
-      normalizeDaemonEvent({
-        id: 54,
-        v: 1,
-        type: 'slow_client_warning',
-        data: {},
-      }),
-    ).toMatchObject([{ type: 'status', text: 'SSE stream is lagging' }]);
+    const warn = vi.spyOn(console, 'warn').mockImplementation(() => undefined);
+    try {
+      expect(
+        normalizeDaemonEvent({
+          id: 54,
+          v: 1,
+          type: 'slow_client_warning',
+          data: { queueSize: 200, maxQueued: 256 },
+        }),
+      ).toEqual([]);
+      expect(warn).toHaveBeenCalledWith('[daemon-ui] SSE stream is lagging', {
+        queueSize: 200,
+        maxQueued: 256,
+      });
+    } finally {
+      warn.mockRestore();
+    }
     expect(
       normalizeDaemonEvent({
         id: 55,