feat(engine): implement async token bucket bandwidth rate limiter

[archittmittal]

Objective & Scope

Implement the asynchronous token bucket rate limiter in src/rate_limiter.py.

Technical Details & Implementation Plan

• Millisecond-resolution async Token Bucket algorithm
• Non-blocking coroutine capacity acquisition via asyncio.sleep()
• Thread-safe / coroutine-safe token replenishment

Deliverable Files

• src/rate_limiter.py
• tests/unit/test_rate_limiter.py

Acceptance Criteria & Checkpoints

• Throughput clamping tests in tests/unit/test_rate_limiter.py within $\pm 3%$ of target

Parent Epic

Part of Epic #39