[kpmgprd101-ops]

Issue for this PR

Closes #58937

Type of change

• Bug fix

What does this PR do?

The Cohere Chat protocol checked part.media.mediaType.startsWith("image/") case-sensitively, so Media.base64(data, "IMAGE/PNG") (or a URL asset declared Image/Png) fell through to the unsupported-content error: Cohere Chat user messages only support text and media content for now. Every other route that classifies media lowercases the type first (anthropic-messages, mistral-chat), which is why they accepted both spellings. The fix lowercases before the prefix check, one line in packages/ai/src/protocols/cohere-chat.ts.

The data URL itself already comes out lowercase (media.ts normalizes it), so no wire-format change.

How did you verify your code works?

• Added a regression test compiling a Cohere request with both an inline IMAGE/PNG base64 image and a URL image declared Image/Png; it asserts both lower to image_url blocks. It fails with the exact error from the issue without the fix and passes with it.
• Full packages/ai suite: 1920 tests, 0 failures.
• bun check (no type errors) and biome check clean on the touched files.

Screenshots / recordings

N/A (protocol compile only).

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[opencode-agent]

Thanks for the fix and the clear write-up, including the regression test for both inline and URL images!

I think this duplicates #58943, which was opened earlier for the same issue (#58937). It changes the same image check in packages/ai/src/protocols/cohere-chat.ts to part.media.kind === "image", which also ignores letter case. It also adds tests for upper-, lower- and mixed-case media types on base64, bytes and URL images. If you think something is missing there, you're very welcome to review or test #58943, or leave a comment on it.