Add support for linux/arm64

Recognize the linux/arm64 platform so the CodeQL CLI is accepted on
Arm64 Linux runners and the native per-platform bundle
codeql-bundle-linux-arm64.tar.zst is resolved, rather than falling
through to the combined all-platforms gzip fallback (which deliberately
omits arm64).

Add ["linux", "arm64"] to the supported platforms and make the linux
bundle name arch-aware. Keep the download mock faithful and cover the
new arch branch with tests.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

src/testing-utils.ts

@@ -936,7 +936,9 @@ export function mockBundleDownloadApi({
     process.platform === "win32"
       ? "win64"
       : process.platform === "linux"
-        ? "linux64"
+        ? process.arch === "arm64"
+          ? "linux-arm64"
+          : "linux64"
         : "osx64";
 
   const baseUrl = apiDetails?.url ?? "https://example.com";