fix: handle missing file gracefully when processing file attachments

[ochen1]

Problem

TL;DR - attach/mention (e.g., @somefile.txt) any file and ask OpenCode to delete it, wait for completion, up arrow, enter.

When a user attached a file (e.g., @somefile.txt) and that file was deleted before submitting the message, Bun.file(filepath).stat() threw an unhandled ENOENT error that printed raw to the TUI, breaking the display: e.g.

ENOENT: no such file or directory, statx '/workspace/ANALYSIS_INDEX.md'
    path: "/workspace/ANALYSIS_INDEX.md",
 syscall: "statx",
   errno: -2,
    code: "ENOENT"

Solution

Wrap the stat() call with .catch(() => null) and handle the missing file case:

1. Publish Session.Event.Error - triggers a toast notification in TUI
2. Add a synthetic text part with the error message - visible to the model
3. Preserve the original file part - user's intent is recorded
   This follows the existing pattern used by ReadTool error handling in the same file.

Testing