Support image, audio, and PDF attachments in all execution modes

[turisanapo]

Support image, audio, and PDF attachments in all execution modes

Request

Support the same attachment types in normal fx sessions and ACP sessions.

Current behavior:

• Normal CLI and TUI sessions support PNG, JPEG, GIF, and WebP images.
• ACP reports that image and audio prompts are unsupported. It rejects image prompt blocks.
• Normal and ACP sessions do not support audio or PDF attachments as model input.

Expected behavior:

• CLI, TUI, and ACP sessions accept images, audio, and PDFs as attachments.
• ACP reports its attachment capabilities accurately.
• fx forwards attachments to the selected provider and surfaces provider errors when an attachment is rejected.

[DarlingtonDeveloper]

Triaged against current main and the open PR set. PR #418 is the current first slice and covers ACP image prompt blocks only. Keeping this issue open for audio and PDF inputs plus attachment parity across CLI, TUI, and ACP.