#24626 cmux NEXT composer: remove the ten-attachment limit

The composer and New Tab accepted only ten attachments because the shared intake, pending chips, kept-chip restore, cloud upload path, and native chat.upload parser all enforced the same count. That self-imposed count is gone: batches and restored chips can exceed ten while per-file type, size, and total-byte safeguards remain. The native parser now accepts eleven-file uploads and still rejects empty lists, invalid names, oversized files, and total payloads over 64 MiB.

Leo said: “the up to ten attachments per message, did we remove that yet? We should just remove it.”

Validation

• git diff --check
• Replaced the old 10-item refusal tests with 11/12-item acceptance coverage for inline attachments and local links.
• Added native AgentPaneUploadTests.acceptsMoreThanTenFilesWithinByteAndNameLimits.
• Native/web test execution is pending the checkout’s fleet-only build/test requirement and the web checkout’s missing @base-ui/utils/useIsoLayoutEffect dependency.

Changelog

Removed the self-imposed ten-attachment composer limit, including the native upload parser cap.

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