macOS: Bundle placeholder Document.icns so Finder can display Zed file icons (#44833)

Generated by AI.

`DocumentTypes.plist` declares `CFBundleTypeIconFile` as `Document` for
Zed’s document types, but the macOS bundle did not include
`Contents/Resources/Document.icns`, causing Finder to fall back to
generic icons.

This PR:
- Adds `crates/zed/resources/Document.icns` as a placeholder document
icon (currently derived from the app icon).
- Updates `script/bundle-mac` to copy it into the `.app` at
`Contents/Resources/Document.icns` during bundling.
- Adds `script/verify-macos-document-icon` for one-command validation.

## How to test (CLI)
1. Build a debug bundle:
   - `./script/bundle-mac -d aarch64-apple-darwin`
2. Verify the bundle contains the referenced icon:
- `./script/verify-macos-document-icon
"target/aarch64-apple-darwin/debug/bundle/osx/Zed Dev.app"`

## Optional visual validation in Finder
- Pick a file (e.g. `.rs`), Get Info → Open with: Zed Dev → Change
All...
- Restart Finder: `killall Finder` (or log out/in)

@JosephTLyons — would you mind running the steps above and confirming
Finder shows Zed’s icon for source files after "Change All" + Finder
restart?

@danilo-leal — this PR ships a placeholder `Document.icns`. When the
real document icon is ready, replace
`crates/zed/resources/Document.icns` and the bundling script will
include it automatically.


Closes #44403.

Release Notes:

- TODO

---------

Co-authored-by: Matt Miller <mattrx@gmail.com>

script/bundle-mac

@@ -106,6 +106,17 @@ mv Cargo.toml.backup Cargo.toml
 popd
 echo "Bundled ${app_path}"
 
+# DocumentTypes.plist references CFBundleTypeIconFile "Document", so the bundle must contain Document.icns.
+# We use the app icon as a placeholder document icon for now.
+document_icon_source="crates/zed/resources/Document.icns"
+document_icon_target="${app_path}/Contents/Resources/Document.icns"
+if [[ -f "${document_icon_source}" ]]; then
+    mkdir -p "$(dirname "${document_icon_target}")"
+    cp "${document_icon_source}" "${document_icon_target}"
+else
+    echo "cargo::warning=Missing ${document_icon_source}; macOS document icons may not appear in Finder."
+fi
+
 if [[ -n "${MACOS_CERTIFICATE:-}" && -n "${MACOS_CERTIFICATE_PASSWORD:-}" && -n "${APPLE_NOTARIZATION_KEY:-}" && -n "${APPLE_NOTARIZATION_KEY_ID:-}" && -n "${APPLE_NOTARIZATION_ISSUER_ID:-}" ]]; then
     can_code_sign=true