[release] Add a dmg target for MacOS (#8207)

Add a dmg target that bundles the codex and codex responses api proxy
binaries for MacOS. this target is signed and notarized.

Verified by triggering a build here:
https://github.com/openai/codex/actions/runs/20318136302/job/58367155205.
Downloaded the artifact and verified that the dmg is signed and
notarized, and the codex binary contained works as expected.

.github/actions/macos-code-sign/notary_helpers.sh

@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+
+notarize_submission() {
+  local label="$1"
+  local path="$2"
+  local notary_key_path="$3"
+
+  if [[ -z "${APPLE_NOTARIZATION_KEY_ID:-}" || -z "${APPLE_NOTARIZATION_ISSUER_ID:-}" ]]; then
+    echo "APPLE_NOTARIZATION_KEY_ID and APPLE_NOTARIZATION_ISSUER_ID are required for notarization"
+    exit 1
+  fi
+
+  if [[ -z "$notary_key_path" || ! -f "$notary_key_path" ]]; then
+    echo "Notary key file $notary_key_path not found"
+    exit 1
+  fi
+
+  if [[ ! -f "$path" ]]; then
+    echo "Notarization payload $path not found"
+    exit 1
+  fi
+
+  local submission_json
+  submission_json=$(xcrun notarytool submit "$path" \
+    --key "$notary_key_path" \
+    --key-id "$APPLE_NOTARIZATION_KEY_ID" \
+    --issuer "$APPLE_NOTARIZATION_ISSUER_ID" \
+    --output-format json \
+    --wait)
+
+  local status submission_id
+  status=$(printf '%s\n' "$submission_json" | jq -r '.status // "Unknown"')
+  submission_id=$(printf '%s\n' "$submission_json" | jq -r '.id // ""')
+
+  if [[ -z "$submission_id" ]]; then
+    echo "Failed to retrieve submission ID for $label"
+    exit 1
+  fi
+
+  echo "::notice title=Notarization::$label submission ${submission_id} completed with status ${status}"
+
+  if [[ "$status" != "Accepted" ]]; then
+    echo "Notarization failed for ${label} (submission ${submission_id}, status ${status})"
+    exit 1
+  fi
+}