This commit fixes all critical and medium severity bugs identified in the
prototype code review:

CRITICAL BUG FIXES:

1. Multi-agent usage_history propagation (Bug pydantic#1)
   - Added usage_history parameter to run(), run_sync(), and iter() methods
   - Pass usage_history when creating GraphAgentState
   - Exposed usage_history in RunContext for tools to access
   - Now tools can pass ctx.usage_history to delegated agents

2. Thread safety for concurrent requests (Bug pydantic#3)
   - Added asyncio.Lock to UsageHistory class
   - Made check_per_minute_limits_before_request() async with locking
   - Made check_per_minute_limits_after_response() async with locking
   - Prevents race conditions where concurrent requests bypass limits

3. Streaming per-minute limit checks (Bug pydantic#2)
   - Updated _get_usage_checking_stream_response() to accept usage_history
   - Added per-minute limit checks during streaming
   - Pass usage_history from AgentStream to checking function
   - Now streaming responses properly enforce per-minute limits

MEDIUM SEVERITY FIXES:

4. Memory leak prevention (Bug pydantic#4)
   - Always cleanup old entries, not just when limits are enabled
   - Prevents unbounded memory growth if limits are toggled

5. Input validation (Bug pydantic#8)
   - Added validation in UsageLimits.__init__() for negative values
   - Raises ValueError if any limit is negative

TESTING:
- All 25 usage limit tests pass
- No regressions in existing tests
- Ready for integration testing with multi-agent scenarios

Changes:
- pydantic_ai/usage.py: Thread safety, validation, async methods
- pydantic_ai/_agent_graph.py: Async await, memory leak fix
- pydantic_ai/_run_context.py: Added usage_history field
- pydantic_ai/agent/abstract.py: Added usage_history parameter
- pydantic_ai/agent/__init__.py: Pass usage_history to state
- pydantic_ai/result.py: Streaming limit checks