Reject unsupported js_repl image MIME types

[etraut-openai]

Summary

codex.emitImage accepted arbitrary image MIME types for byte payloads and data URLs. That allowed a value like image/rgba to be wrapped as an input_image, even though it is not a supported encoded image format, so the invalid image could reach the model-input path and trigger output sanitization.

This results in a panic in debug builds because the output sanitization is meant as a final safety net, not a primary means of rejecting invalid image types. I've hit this case multiple times when executing certain long-running tasks.

This PR rejects unsupported image MIME types before they are emitted from js_repl.

Changes

• Validate codex.emitImage({ bytes, mimeType }) in the JS kernel so only encoded PNG, JPEG, WebP, or GIF payloads are accepted.
• Apply the same MIME allowlist to direct image data URLs, including the Rust host-side validation path.
• Clarify the JS REPL instructions so agents know byte payloads must already be encoded as PNG/JPEG/WebP/GIF.

[bolinfest]

I don't feel strongly about any of these comments, FYI!

[bolinfest]

I would consider naming assertEmitImageMimeType() to underscore the fact that this will throw.

I'm so used to Rust: should we return something Result-like instead of throwing? I'm not familiar with how we handle error-handling in this code.

[bolinfest]

Is this where the 5 comes from?

Suggested change

	const mediaType = dataUrl.slice(5, commaIndex).split(";")[0];
	const mediaType = dataUrl.slice('data:'.length, commaIndex).split(";")[0];

[bolinfest]

Alternative to the repeated !==?

Suggested change

	if (
	normalized !== "image/png" &&
	normalized !== "image/jpeg" &&
	normalized !== "image/webp" &&
	normalized !== "image/gif"
	) {
	throw new Error(
	"codex.emitImage only supports image/png, image/jpeg, image/webp, or image/gif",
	);
	}
	const SUPPORTED_IMAGE_TYPES = [
	"image/png",
	"image/jpeg",
	"image/webp",
	"image/gif",
	];
	if (!SUPPORTED_IMAGE_TYPES.includes(normalized)) {
	throw new Error(
	`codex.emitImage only supports ${SUPPORTED_IMAGE_TYPES.join(", ")}`,
	);
	}

[bolinfest]

strip_prefix() to the rescue!

Suggested change

	if !image_url
	.get(..5)
	.is_some_and(|scheme| scheme.eq_ignore_ascii_case("data:"))
	{
	return Err("codex.emitImage only accepts data URLs".to_string());
	}
	let data_payload = image_url.strip_prefix("data:").ok_or_else(|| anyhow!("expected data: URI but got `{image_url}`"))?;

Oh, but you need to support eq_ignore_ascii_case()? Is that a thing for URIs?