An MCP tool's ImageContent was cached and the model got back only a
`MEDIA:/path` line, so a vision main model could never read the image
(a raw cua-driver MCP server could not see the screen at all).
The registry handler now returns the `_multimodal` envelope when the
active route takes images inside tool results, decided by the same gate
vision_analyze and browser screenshots use (`agent.image_input_mode`, an
explicit `auxiliary.vision` backend, catalog vision, provider support).
The image is re-encoded from the sniffed cache file, normalized to a
provider-accepted format and sized to `vision.embed_target_bytes` /
1568 px (it is re-sent every later turn), at most 4 per result; the text
half keeps the MEDIA: paths. Paths come from this call's own cache
writes, never parsed out of server text. The resize runs on the tool
thread, off the shared MCP loop. Plugins' `ctx.call_mcp` keeps its
string contract (native_images is set only on the registry handler).
Builds on #131374 by @sarahwu185, which reused the computer_use capture
gate (ignores image_input_mode, native for catalog text-only models) and
inlined the server's raw base64 unvalidated and unresized.
6da034a60d