fix(computer_use): preserve app context for capture_after; fix element label parsing (#24170 bugs 2 & 5)

Bug 2 (capture_after=True loses app context):
_maybe_follow_capture called backend.capture(mode='som') with no app=,
causing cua-driver to capture the frontmost window instead of the app
targeted by the preceding capture/focus_app. Fix: track _last_app on
CuaDriverBackend and thread it through the follow-up capture call so
the same app is re-captured regardless of which window has OS focus.

Bug 5 (element labels stripped in capture results):
_ELEMENT_LINE_RE matched the classic '  - [N] AXRole "label"' format
but not the '[N] AXRole (order) id=Label' format introduced in
cua-driver v0.1.6. All element labels were silently dropped as empty
strings, making element identification impossible.

Fix: extend regex to capture both group(3) (quoted label) and group(4)
(id= label), and update _parse_elements_from_tree to use group(4) as
fallback. Both old and new cua-driver output now produce populated
UIElement.label values.

focus_app() now also sets _last_app so that capture_after= on any
subsequent action re-targets the focused app.

5 new regression tests added.

Part of #24170 (bugs 1 and 3/4 addressed separately).

tools/computer_use/tool.py

@@ -463,7 +463,11 @@ def _maybe_follow_capture(
     if not do_capture:
         return _text_response(res)
     try:
-        cap = backend.capture(mode="som")
+        # Preserve the app context established by the preceding capture/focus_app so
+        # that capture_after=True re-captures the same app rather than the frontmost
+        # window (which may have changed if the action caused a focus shift).
+        last_app = getattr(backend, "_last_app", None)
+        cap = backend.capture(mode="som", app=last_app)
     except Exception as e:
         logger.warning("follow-up capture failed: %s", e)
         return _text_response(res)