refactor(agent-core): unify image extension sniff-failed detection

[7Sageer]

Related Issue

Follow-up to #970 (review feedback); no separate issue.

Problem

detectFileType expressed the rule "an image extension without confirming magic is not an image" twice — once in the media-mode branch and once in the text-mode fall-through — each with its own copy of the same comment. The rule is mode-independent, so the duplication forces any future change to this path to be edited in two places, or the two modes silently diverge.

Two test gaps from #970 are also closed here: the text-mode behavior change (an image-extension file whose bytes are not an image is reported as unknown rather than image) was not covered by the Read tool tests, and the matching file-type test only exercised text mode while its comment described media mode.

What changed

• Merge the two duplicate image-extension guards into a single mode-independent check, keeping the video extension fallback before the NUL check so video containers with no magic still win. No behavior change.
• Add a Read tool test: an image-extension file with non-image bytes is rejected as "not readable" instead of being redirected to ReadMediaFile.
• Align the file-type test to assert the sniff-failed image case in both media and text modes, matching its comment.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

🦋 Changeset detected

Latest commit: d866b78

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Patch

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commit: d866b78