Harden agent attachment drops

[udaychandra]

Summary

• Keep existing Mast agent thread image/PDF drop support, but make image-looking files validate at the attachment boundary instead of silently falling back to file mentions.
• Apply the existing 25 MB external attachment limit to known raster image attachments.
• Surface invalid and oversized image attachment errors through workspace notifications while preserving normal text/code file drop behavior.

Verification

• cargo fmt --all -- --check
• cargo check -p agent_ui
• cargo test -p agent_ui external_image
• cargo test -p agent_ui test_paste_external_pdf_path_inserts_blob_mention

Release Notes:

• Improved Mast agent thread attachment handling for dragged or pasted image/PDF files.