Default MCP tab tools to the active tab

Requiring an explicit tab_id forced every navigate or close through a
preceding tabs_list call, which costs an MCP client a full round trip just to
learn the ID the browser already considers current.

navigate and tabs_close now resolve the active tab when tab_id is omitted,
matching the screenshot tool. Resolution is explicit rather than forwarding
None into the SDK, so the acting tool knows which tab it touched; tabs_close
reports it, since closing the wrong tab is not recoverable.

This stays in the MCP layer: the SDK and CLI signatures are unchanged.

tests/test_mcp.py

@@ -26,6 +26,7 @@ class FakeClient:
       list=self.tabs_list,
       open=self.tabs_open,
       close=self.tabs_close,
+      active=self.tabs_active,
       status=self.tabs_status,
       screenshot=self.tabs_screenshot,
     )
@@ -46,6 +47,10 @@ class FakeClient:
     self.calls.append(("close", tab_id))
     return 1
 
+  def tabs_active(self):
+    self.calls.append(("active",))
+    return SimpleNamespace(id=7)
+
   def tabs_status(self, tab_id):
     self.calls.append(("status", tab_id))
     return {"id": tab_id, "title": "Navigated", "url": "https://example.com/next"}
@@ -178,6 +183,18 @@ async def test_mutating_tools_use_sdk_and_return_fresh_state(client):
     ("navigate", 8, "https://example.com/next"), ("status", 8)
   ]
 
+@pytest.mark.anyio
+async def test_tab_tools_default_to_the_active_tab(client):
+  navigated = await client.call_tool("browser_navigate", {"url": "https://example.com/next"})
+  closed = await client.call_tool("browser_tabs_close", {})
+
+  assert navigated.structured_content["id"] == 7
+  assert closed.structured_content == {"closed": 1, "tab_id": 7}
+  assert FakeClient.instances[0].calls == [
+    ("active",), ("navigate", 7, "https://example.com/next"), ("status", 7)
+  ]
+  assert FakeClient.instances[1].calls == [("active",), ("close", 7)]
+
 @pytest.mark.anyio
 async def test_screenshot_returns_image_content(client):
   result = await client.call_tool("browser_screenshot", {"tab_id": 7, "format": "png"})