fix(computer_use): use get_window_state for vision/som captures (#39242)

[kyssta-exe]

Fixes #39242.

Replace the non-existent cua-driver screenshot tool call with get_window_state(capture_mode="vision") for vision-mode captures, and pass capture_mode to get_window_state for som/ax modes so SOM captures also get their screenshot payload back.

Changes in tools/computer_use/cua_backend.py:

• Vision mode: calls get_window_state with capture_mode="vision" instead of the non-existent screenshot tool; extracts width/height from structuredContent.
• Non-vision modes: passes capture_mode ("som" or "ax") to get_window_state so SOM captures include the screenshot image alongside the AX tree.
• Updated docstring to reflect the corrected tool mapping.

[alt-glitch]

Duplicate of #39262, which makes the same fix (route cua-driver vision/SOM captures through get_window_state(capture_mode=...) instead of the non-existent screenshot tool) for the same issue #39242.

[alaamohanad169-ship-it]

Note: This is NOT a duplicate of #39262. The referenced PRs were closed/superseded earlier. This version has been cherry-picked onto the latest origin/main with a clean diff containing only the fix changes. No conflicts. CI green. Ready for review.

[teknium1]

Thanks for the focused report and patch. The underlying cua-driver vision-capture regression is already fixed on current main, so this PR is now redundant.

• tools/computer_use/cua_backend.py:1373–1418 uses get_window_state when a modern driver no longer advertises screenshot, while retaining compatibility with older drivers.
• tests/tools/test_computer_use.py:2360–2407 covers the dropped-screenshot case and verifies that vision capture returns a PNG with no AX-element noise.
• The implementation shipped via merge commit 760fd9513e7d7350a672c76369ba632ca1ed1448 (PR fix(computer-use): vision capture returns an image on cua-driver >=0.5.x #51078), included in v2026.7.1.

Automated hermes-sweeper review.