Fix: gracefully error out for unsupported images (#7478)

Fix for #7459 
## What
Since codex errors out for unsupported images, stop attempting to
base64/attach them and instead emit a clear placeholder when the file
isn’t a supported image MIME.

## Why
Local uploads for unsupported formats (e.g., SVG/GIF/etc.) were
dead-ending after decode failures because of the 400 retry loop. Users
now get an explicit “cannot attach … unsupported image format …”
response.

## How
Replace the fallback read/encode path with MIME detection that bails out
for non-image or unsupported image types, returning a consistent
placeholder. Unreadable and invalid images still produce their existing
error placeholders.

codex-rs/Cargo.lock

@@ -1491,7 +1491,6 @@ name = "codex-protocol"
 version = "0.0.0"
 dependencies = [
  "anyhow",
- "base64",
  "codex-git",
  "codex-utils-image",
  "icu_decimal",