feat: bootstrap bun toolchain skeleton and smoke test

tests/toolchain_test/BUILD.bazel

@@ -0,0 +1,10 @@
+sh_test(
+    name = "bun_version_test",
+    srcs = ["toolchain_version.sh"],
+    args = ["$(location @bun_linux_x64//:bun)"],
+    data = ["@bun_linux_x64//:bun"],
+    target_compatible_with = [
+        "@platforms//cpu:x86_64",
+        "@platforms//os:linux",
+    ],
+)

tests/toolchain_test/toolchain_version.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+bun_path="$1"
+version="$(${bun_path} --version)"
+
+if [[ ! "${version}" =~ ^[0-9]+\.[0-9]+\.[0-9]+ ]]; then
+  echo "Unexpected bun version output: ${version}" >&2
+  exit 1
+fi