fix(e2e): use gateway sandbox image in TypeScript exec test

[drew]

Summary

Make the TypeScript interactive-exec E2E scenario create its sandbox from the same image selected by the Docker gateway wrapper.

Related Issue

Localized stability fix found during the E2E campaign stacked on #3583; no separate issue is needed.

Changes

• Export the wrapper's resolved sandbox image to its test command. This covers the wrapper default as well as explicit image overrides.
• Document the image contract in the build architecture notes.

Testing

• mise run e2e:sdk:ts:exec — passed locally; one interactive-exec scenario passed against nvcr.io/nvidia/base/ubuntu:24.04.
• mise run pre-commit — passed.
• mise run ci — passed.

Checklist

• Signed-off commit and pre-commit checks
• Architecture documentation updated
• Full local CI passed

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[drew]

Closing to narrow the automated flake investigation to the four selected fixes (#3583/#3588, #3598, #3604, #3608). The findings remain available here for later follow-up.