fix(test): reduce thread count on macOS CI to prevent SIGABRT

macOS CI runners have limited memory compared to Linux runners. Using
os.cpus().length threads was causing memory exhaustion and SIGABRT
errors during test execution.

Changes:
- Detect macOS CI environment (CI=true && platform=darwin)
- Use 50% of CPUs on macOS CI (min 2) to prevent memory issues
- Keep full CPU count on other platforms for optimal performance
- Update minThreads calculation to be proportional to maxThreads

This should resolve the intermittent SIGABRT failures on macOS CI
while maintaining full performance on Linux and Windows runners.

Author: jdalton

packages/cli/vitest.config.mts

@@ -14,6 +14,23 @@ const isCoverageEnabled =
   process.env.npm_lifecycle_event === 'cover' ||
   process.argv.includes('--coverage')
 
+// Detect if running in CI on macOS.
+const isMacCI = 'CI' in process.env && process.platform === 'darwin'
+
+// Calculate optimal thread count based on environment.
+// macOS CI runners have limited memory, so use fewer threads to prevent SIGABRT.
+function getMaxThreads(): number {
+  if (isCoverageEnabled) {
+    return 1
+  }
+  if (isMacCI) {
+    // Use 50% of CPUs on macOS CI to prevent memory exhaustion.
+    return Math.max(2, Math.floor(os.cpus().length / 2))
+  }
+  // Use all CPUs on other platforms.
+  return os.cpus().length
+}
+
 export default defineConfig({
   resolve: {
     alias: getLocalPackageAliases(repoRoot),
@@ -40,8 +57,9 @@ export default defineConfig({
         singleThread: false,
         // Maximize parallel execution to offset isolate: true performance cost.
         // Use CPU count for better hardware utilization.
-        maxThreads: isCoverageEnabled ? 1 : os.cpus().length,
-        minThreads: isCoverageEnabled ? 1 : Math.min(4, os.cpus().length),
+        // Reduce threads on macOS CI to prevent memory exhaustion (SIGABRT).
+        maxThreads: getMaxThreads(),
+        minThreads: isCoverageEnabled ? 1 : Math.min(2, Math.floor(getMaxThreads() / 2)),
         // IMPORTANT: Changed to isolate: true to fix worker thread termination issues.
         //
         // Previous configuration (isolate: false) caused "Terminating worker thread"