Add Buildkite job to run cross-platform Swift package tests

Runs `swift test` from the repository root to verify that cross-platform
modules build and pass tests on macOS without Xcode.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: jkmassel

.buildkite/commands/run-swift-package-tests.sh

@@ -0,0 +1,8 @@
+#!/bin/bash -eu
+
+if "$(dirname "${BASH_SOURCE[0]}")/should-skip-job.sh" --job-type validation; then
+  exit 0
+fi
+
+echo "--- :swift: Running cross-platform Swift package tests"
+swift test

.buildkite/pipeline.yml

@@ -58,6 +58,13 @@ steps:
   - group: "🔬 Unit Tests"
     key: unit_tests_group
     steps:
+      - label: "🔬 :swift: Swift Package Tests"
+        command: .buildkite/commands/run-swift-package-tests.sh
+        plugins: [$CI_TOOLKIT_PLUGIN]
+        notify:
+          - github_commit_status:
+              context: "Swift Package Tests"
+
       - label: "🔬 :wordpress: Unit Tests"
         command: ".buildkite/commands/run-unit-tests.sh"
         depends_on: "build_wordpress"