fix(computer-use): skip capture_after when action failed (ok=False)

_maybe_follow_capture() issued a follow-up screenshot unconditionally
when capture_after=True, even when res.ok=False. The model then received
a normal-looking screenshot alongside an error message, and in practice
it often ignored ok=False and proceeded as if the action had succeeded.

Fix: return _text_response(res) early when res.ok is False so the model
receives only the error and can decide how to recover.

Tests added:
- test_capture_after_skipped_when_action_failed: patches click to return
  ok=False and asserts no capture call is issued.
- test_capture_after_fires_when_action_succeeds: ensures the happy path
  still triggers the follow-up capture.

tools/computer_use/tool.py

@@ -675,6 +675,11 @@ def _maybe_follow_capture(
 ) -> Any:
     if not do_capture:
         return _text_response(res)
+    # Skip the follow-up capture when the action itself failed: showing a
+    # normal-looking screenshot after a failure misleads the model into thinking
+    # the action succeeded. Return the error text instead.
+    if not res.ok:
+        return _text_response(res)
     try:
         # 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