Vision mode called a `screenshot` MCP tool that cua-driver dropped in
0.5.x (full-window PNG capture was folded into `get_window_state`). The
driver replied "Unknown tool: screenshot", so `images` came back empty,
`png_b64` stayed None, and capture returned a 0x0 result with no image
on every call. `som`/`ax` were unaffected because they already use
`get_window_state`, which masked the regression.
Route vision by capability:
- driver advertises `screenshot` (older builds) -> use it (no AX walk)
- otherwise -> call `get_window_state` but discard the AX tree/elements,
returning only the PNG so vision stays free of element noise
- capabilities not yet discovered -> try `screenshot`, fall back to
`get_window_state` on an empty image, so the path self-heals
Add `_image_from_tool_result` to pull the PNG from either an MCP image
content-part or `structuredContent.screenshot_png_b64`, and use it on
the som path too so the image won't silently drop on driver builds that
deliver it via structuredContent instead of a content part.
Verified live (vision: 1568x954, 0 elements; som: image + 527 elements)
and with unit coverage of all four routing cases.
807b696295