feat: add prompt caching, sticky account selection, and non-thinking model

- Implement sticky account selection for prompt cache continuity
- Derive stable session ID from first user message (SHA256 hash)
- Return cache_read_input_tokens in usage metadata
- Add claude-sonnet-4-5 model without thinking
- Remove DEFAULT_THINKING_BUDGET (let API use its default)
- Add prompt caching test
- Update README and CLAUDE.md documentation

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

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

tests/run-all.cjs

@@ -13,7 +13,8 @@ const tests = [
     { name: 'Multi-turn Tools (Non-Streaming)', file: 'test-multiturn-thinking-tools.cjs' },
     { name: 'Multi-turn Tools (Streaming)', file: 'test-multiturn-thinking-tools-streaming.cjs' },
     { name: 'Interleaved Thinking', file: 'test-interleaved-thinking.cjs' },
-    { name: 'Image Support', file: 'test-images.cjs' }
+    { name: 'Image Support', file: 'test-images.cjs' },
+    { name: 'Prompt Caching', file: 'test-caching-streaming.cjs' }
 ];
 
 async function runTest(test) {