### Motivation
- Prevent untrusted js_repl code from supplying arbitrary external URLs
that the host would forward into model input and cause external fetches
/ data exfiltration. This change narrows the emitImage contract to safe,
self-contained data URLs.
### Description
- Kernel: added `normalizeEmitImageUrl` and enforce that string-valued
`codex.emitImage(...)` inputs and `input_image`/content-item paths only
accept non-empty `data:` URLs; byte-based paths still produce data URLs
as before (`kernel.js`).
- Host: added `validate_emitted_image_url` and check `EmitImage`
requests before creating `FunctionCallOutputContentItem::InputImage`,
returning an error to the kernel if the URL is not a `data:` URL
(`mod.rs`).
- Tests/docs: added a runtime test
`js_repl_emit_image_rejects_non_data_url` to assert rejection of
non-data URLs and updated user-facing docs/instruction text to state
`data URL` support instead of generic direct image URLs (`mod.rs`,
`docs/js_repl.md`, `project_doc.rs`).
### Testing
- Ran `just fmt` in `codex-rs`; it completed successfully.
- Added a runtime test (`cargo test -p codex-core
js_repl_emit_image_rejects_non_data_url`) but executing the test in this
environment failed due to a missing system dependency required by
`codex-linux-sandbox` (the vendored `bubblewrap` build requires
`libcap.pc` via `pkg-config`), so the test could not be run here.
- Attempted a focused `cargo test` invocation with and without default
features; both compile/test attempts were blocked by the same missing
system `libcap` dependency in this environment.
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[Codex
Task](https://chatgpt.com/codex/tasks/task_i_69a7837bce98832d91db92d5f76d6cbe)
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