fix(ui): unread badge latches un-clearable when the read patch is never sent (#123252)

sessions.patch resolves null without sending a request when the
connection scope cannot be captured, but the read-marker guard only
unlatched on promise rejection — so the once-per-episode latch stayed
set with no request in flight, and the unread badge stayed lit on the
open session until navigating away and back. Treat a null resolution
like a failure.

ui/src/pages/chat/chat-pane-session.ts

@@ -189,11 +189,20 @@ export abstract class ChatPaneSession extends ChatPaneTaskSuggestions {
       return;
     }
     const guardKey = state.sessionKey;
-    void this.context.sessions.patch(row.key, { unread: false }, { agentId }).catch(() => {
-      // Unlatch so later unread snapshots retry; the session capability
-      // publishes the actionable error for the owning page.
-      this.unreadPatchGuard.patchFailed(guardKey);
-    });
+    void this.context.sessions.patch(row.key, { unread: false }, { agentId }).then(
+      (result) => {
+        // A null result means no request was sent (connection scope lost);
+        // unlatch like a failure or the badge stays lit until navigation.
+        if (result === null) {
+          this.unreadPatchGuard.patchFailed(guardKey);
+        }
+      },
+      () => {
+        // Unlatch so later unread snapshots retry; the session capability
+        // publishes the actionable error for the owning page.
+        this.unreadPatchGuard.patchFailed(guardKey);
+      },
+    );
   }
 
   protected async restoreArchivedSession(sessionKey: string, expectedSessionId: string) {

ui/src/pages/chat/chat-pane.read-marker.test.ts

@@ -90,6 +90,30 @@ describe("chat pane read markers", () => {
     expect(state.lastError).toBeNull();
   });
 
+  it("retries the read patch after a null (unsent) resolution", async () => {
+    // sessions.patch resolves null without a request when the connection
+    // scope is lost; the guard must unlatch like a failure or the badge
+    // stays lit until navigation.
+    const patch = vi.fn().mockResolvedValue(null);
+    const { pane } = createTestChatPane({
+      client: {} as GatewayBrowserClient,
+      sessions: { patch } as unknown as SessionCapability,
+    });
+    const row = {
+      key: "agent:main:current",
+      kind: "direct" as const,
+      label: "Unread",
+      updatedAt: 20,
+      unread: true,
+    };
+
+    pane.markSessionRead(row);
+    await Promise.resolve();
+    pane.markSessionRead(row);
+
+    expect(patch).toHaveBeenCalledTimes(2);
+  });
+
   it("does not clear unread from a hidden retained pane", () => {
     const patch = vi.fn().mockResolvedValue(null);
     const { pane } = createTestChatPane({