fix(browser): keep snapshot refs on server browsers that use an external Chromium (#36276)

Every command re-selected its tab first, and agent-browser drops the last
snapshot's refs on any tab select, so each @ref failed as Unknown ref.
Select the tab only when it is not already active.

src/main/orcad/external-chromium-browser-session.test.ts

@@ -167,6 +167,32 @@ describe('orcad external-chromium agent-browser environment', () => {
     expect(issued).not.toContain('open')
   })
 
+  it('selects a tab only when it is not already active, keeping the snapshot refs', async () => {
+    runProcessMock.mockImplementation(() => {
+      const tabs = [
+        { active: true, tabId: 't1', title: 'a', url: 'about:blank' },
+        { active: false, tabId: 't2', title: 'b', url: 'about:blank' }
+      ]
+      return Promise.resolve({
+        code: 0,
+        signal: null,
+        stdout: JSON.stringify({ success: true, data: { tabs } }),
+        stderr: '',
+        timedOut: false
+      })
+    })
+    const session = new ExternalChromiumBrowserSession(
+      '/opt/orca/agent-browser',
+      { executablePath: BASE.executablePath, provider: 'chromium' },
+      '/state'
+    )
+
+    await session.selectPage('t1')
+    expect(commands()).toEqual([['tab']])
+    await session.selectPage('t2')
+    expect(commands()).toEqual([['tab'], ['tab'], ['tab', 't2']])
+  })
+
   it('reclaims a session that answers nothing, then opens', async () => {
     let listed = 0
     runProcessMock.mockImplementation((spec: Spec) => {

src/main/orcad/external-chromium-browser-session.ts

@@ -138,6 +138,10 @@ export class ExternalChromiumBrowserSession {
   }
 
   async selectPage(agentPageId: string): Promise<void> {
+    // Selecting any tab, even the active one, discards the refs of the last snapshot.
+    if ((await this.readTabs()).some((tab) => tab.tabId === agentPageId && tab.active)) {
+      return
+    }
     await this.run(['tab', agentPageId])
   }