fix(acp): preserve file attachment metadata during session replay

[liorshk]

Problem

When reloading an ACP session, file attachments were not being replayed correctly:

• file type parts in stored messages were not being sent to ACP clients at all
• Images lost their original filenames (always showed as "image")
• Binary files (PDFs, etc.) were not replayed
• Text files appeared as inline text instead of as file attachments
• Synthetic text parts (injected for LLM context) were incorrectly sent to ACP clients

Solution

Updated processMessage() in agent.ts to properly handle file parts during session replay:

• file:// URLs → Replayed as resource_link blocks
• Images → Replayed as image blocks with preserved original filename
• Text files (text/*, application/json) → Replayed as resource blocks with decoded text
• Binary files → Replayed as resource blocks with blob data

Also updated the prompt() method to:

• Preserve original filenames from resource_link.name and image.uri
• Handle binary resource blocks (PDFs, etc.) by storing them as file parts with data URLs
• Check for truthy resource.text instead of just property existence

Changes

• Add file part handling in processMessage() for session replay
• Filter synthetic text parts from ACP output (!part.synthetic)
• Preserve filename from resource_link.name when converting to internal parts
• Handle binary resource.blob blocks in prompt()

[rekram1-node]

im skeptical of the updates here, what are the reasons here?

This file shouldnt really need updates to fix acp stuff

[liorshk]

You're right - the message-v2.ts changes were unnecessary for this fix.
The initial approach tried to handle binary content detection at the LLM message conversion layer, but that was the wrong place to solve this problem. The actual issue is purely in the ACP layer: when replaying a session, OpenCode's file parts weren't being converted back to proper ACP content blocks (image, resource, resource_link).
The current fix keeps all changes in agent.ts.

I updated the PR and the PR description.

[rekram1-node]

/review

[github-actions]

Minor style note: This uses a let statement which the style guide recommends avoiding. One alternative would be to calculate the effective MIME type inline:

const effectiveMime = (() => {
  const base64Match = url.match(/^data:([^;]+);base64,(.*)$/)
  return base64Match?.[1] || part.mime || "application/octet-stream"
})()

However, the current approach is readable and the reassignment is simple enough. Feel free to keep as-is if you prefer the clarity.

[github-actions]

lgtm