chore: add empty response retry test and fix flaky tests

- Add test:emptyretry script and include in test suite
- Fix test-interleaved-thinking: use complex prompt to force thinking
- Fix test-multiturn-thinking-tools: make Turn 2 lenient (thinking optional)
- Fix test-multiturn-thinking-tools-streaming: same lenient approach
- Use TEST_MODELS helper instead of hardcoded model ID

Models may skip thinking on obvious next steps - this is valid behavior.
Tests now only require thinking on first turn to verify signatures work.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/run-all.cjs

@@ -16,7 +16,8 @@ const tests = [
     { name: 'Image Support', file: 'test-images.cjs' },
     { name: 'Prompt Caching', file: 'test-caching-streaming.cjs' },
     { name: 'Cross-Model Thinking', file: 'test-cross-model-thinking.cjs' },
-    { name: 'OAuth No-Browser Mode', file: 'test-oauth-no-browser.cjs' }
+    { name: 'OAuth No-Browser Mode', file: 'test-oauth-no-browser.cjs' },
+    { name: 'Empty Response Retry', file: 'test-empty-response-retry.cjs' }
 ];
 
 async function runTest(test) {