core: resolve view_image paths in selected environment (#29526)

## Why

view_image needs to support foreign OS remote executors.

## What

- resolve image paths against the selected environment as `PathUri` and
read them through that environment's filesystem
- keep app-server's public path field wire-compatible as
`LegacyAppPathString`, with purpose-specific UI rendering
- cover relative and absolute target-native paths in the core
integration test and run the full `view_image` suite under wine-exec
without skips

codex-rs/app-server-protocol/schema/json/v2/ThreadMetadataUpdateResponse.json

@@ -1562,7 +1562,7 @@
               "type": "string"
             },
             "path": {
-              "$ref": "#/definitions/AbsolutePathBuf"
+              "$ref": "#/definitions/LegacyAppPathString"
             },
             "type": {
               "enum": [