fix(browser): remove premature browser cleanup after subagent invocation (#24753)

integration-tests/browser-agent.persistent-session.responses

@@ -0,0 +1,8 @@
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"text":"I'll browse to example.com twice to verify the content. Let me first check the page title, then check the links on the page."},{"functionCall":{"name":"browser_agent","args":{"task":"Navigate to https://example.com and tell me the page title using the accessibility tree"}}}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":100,"candidatesTokenCount":50,"totalTokenCount":150}}]}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"functionCall":{"name":"navigate_page","args":{"url":"https://example.com"}}}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":100,"candidatesTokenCount":30,"totalTokenCount":130}}]}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"functionCall":{"name":"take_snapshot","args":{}}}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":150,"candidatesTokenCount":20,"totalTokenCount":170}}]}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"functionCall":{"name":"complete_task","args":{"result":{"success":true,"summary":"Page title is 'Example Domain'."}}}}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":200,"candidatesTokenCount":40,"totalTokenCount":240}}]}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"text":"The page title is 'Example Domain'. Now let me check the links on the page."},{"functionCall":{"name":"browser_agent","args":{"task":"Take a snapshot of the accessibility tree on the currently open page and tell me about any links"}}}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":200,"candidatesTokenCount":50,"totalTokenCount":250}}]}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"functionCall":{"name":"take_snapshot","args":{}}}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":150,"candidatesTokenCount":20,"totalTokenCount":170}}]}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"functionCall":{"name":"complete_task","args":{"result":{"success":true,"summary":"Found a link 'More information...' pointing to iana.org."}}}}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":200,"candidatesTokenCount":40,"totalTokenCount":240}}]}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"text":"I browsed example.com twice using persistent browser sessions:\n\n1. **First visit**: Page title is 'Example Domain'\n2. **Second visit**: Found a link 'More information...' pointing to iana.org\n\nThe browser stayed open between both visits, confirming persistent session management works correctly."}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":300,"candidatesTokenCount":60,"totalTokenCount":360}}]}

integration-tests/browser-agent.test.ts

@@ -229,6 +229,51 @@ describe.skipIf(!chromeAvailable)('browser-agent', () => {
     assertModelHasOutput(result);
   });
 
+  it('should keep browser open across multiple browser_agent invocations', async () => {
+    rig.setup('browser-persistent-session', {
+      fakeResponsesPath: join(
+        __dirname,
+        'browser-agent.persistent-session.responses',
+      ),
+      settings: {
+        agents: {
+          overrides: {
+            browser_agent: {
+              enabled: true,
+            },
+          },
+          browser: {
+            headless: true,
+            sessionMode: 'isolated',
+          },
+        },
+      },
+    });
+
+    const result = await rig.run({
+      args: 'Browse to example.com twice: first get the page title, then check for links.',
+    });
+
+    const toolLogs = rig.readToolLogs();
+    const browserCalls = toolLogs.filter(
+      (t) => t.toolRequest.name === 'browser_agent',
+    );
+
+    // Both browser_agent invocations must succeed — if the browser was
+    // incorrectly closed after the first call (regression #24210),
+    // the second call would fail.
+    expect(
+      browserCalls.length,
+      'Expected browser_agent to be called twice',
+    ).toBe(2);
+    expect(
+      browserCalls.every((c) => c.toolRequest.success),
+      'Both browser_agent calls should succeed',
+    ).toBe(true);
+
+    assertModelHasOutput(result);
+  });
+
   it('should handle tool confirmation for write_file without crashing', async () => {
     rig.setup('tool-confirmation', {
       fakeResponsesPath: join(