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>

package.json

@@ -27,7 +27,8 @@
     "test:images": "node tests/test-images.cjs",
     "test:caching": "node tests/test-caching-streaming.cjs",
     "test:crossmodel": "node tests/test-cross-model-thinking.cjs",
-    "test:oauth": "node tests/test-oauth-no-browser.cjs"
+    "test:oauth": "node tests/test-oauth-no-browser.cjs",
+    "test:emptyretry": "node tests/test-empty-response-retry.cjs"
   },
   "keywords": [
     "claude",