feat: add lane-based priority preemption test examples (Rust/Python/Node.js)

Rewrote test_task_priority examples to demonstrate real cross-lane priority
preemption using the lane-based scheduling system:

- Test 1: Query (P1) preempts queued Execute (P2) tasks
- Test 2: Multi-level priority ordering (Query vs Execute)
- Test 3: Late urgent task insertion into busy queue

Each test constrains lane concurrency to create visible scheduling bottlenecks,
submits low-priority Execute-lane tasks first, then injects high-priority
Query-lane tasks and tracks completion order with timestamps.

All three SDKs use Arc<AgentSession> / asyncio.create_task / Promise.allSettled
for concurrent task submission with real LLM calls.

Author: ZhiXiao-Lin