fix: mock sleep in retry tests to prevent OOM + use single thread

1. Mocked client.sleep() in mcp-client-core-part2.test.js error counting tests
   - Tests were taking 3005ms each with real retries (1000ms + 2000ms delays)
   - Now take ~1ms with mocked sleep
   - Reduces async work and memory pressure

2. Changed vitest config to singleThread: true
   - Reduces worker thread memory pressure
   - Tests still pass, just slightly slower

The OOM issue is caused by cumulative memory pressure from 14 test files
running together. Even with these fixes, OOM still occurs but is improved.

Further investigation needed - may need to split more files or increase
Node memory limit in CI.

Co-authored-by: pethers <1726836+pethers@users.noreply.github.com>

Author: Copilot

tests/mcp-client-core-part2.test.js

@@ -365,6 +365,9 @@ describe('MCPClient Part 2', () => {
 
   describe('error counting accuracy', () => {
     it('should not over-count errors on retried requests', async () => {
+      // Mock sleep to make test fast (prevent 3000ms delay causing OOM)
+      vi.spyOn(client, 'sleep').mockResolvedValue();
+      
       let attempt = 0;
       global.fetch = vi.fn(() => {
         attempt++;
@@ -386,6 +389,9 @@ describe('MCPClient Part 2', () => {
     });
 
     it('should count exactly one error for a fully failed request', async () => {
+      // Mock sleep to make test fast (prevent 3000ms delay causing OOM)
+      vi.spyOn(client, 'sleep').mockResolvedValue();
+      
       global.fetch = vi.fn(() => Promise.reject(new Error('Network error')));
 
       try {

vitest.config.js

@@ -108,7 +108,7 @@ export default defineConfig({
 
     // Pool options for parallel execution (Vitest 4+)
     pool: 'threads',
-    singleThread: false,
+    singleThread: true, // Single thread to reduce memory pressure
 
     // Mock configuration
     mockReset: true,