fix: Resolve RuntimeError on async generator cleanup

[wingding12]

Summary

This PR fixes issue #454 where a RuntimeError: Attempted to exit cancel scope in a different task than it was entered in was raised during async generator cleanup.

Problem

The issue occurred when:

1. Query.start() called __aenter__() on the task group in task A
2. When an async generator was cleaned up (e.g., breaking from async for loop, or when asyncio.run() shuts down), Query.close() called __aexit__() in task B
3. AnyIO requires cancel scope enter/exit to happen in the same task

Solution

1. Added CancelScope for reader task - The reader task now has its own CancelScope that can be safely cancelled from any task context, without requiring the task group's __aexit__().

2. Graceful __aexit__() handling - The close() method now catches the RuntimeError when __aexit__() is called from a different task and handles it gracefully by logging and continuing cleanup.

3. Added aclosing() wrapper - process_query() now uses contextlib.aclosing() for better async generator cleanup.

4. Query as async context manager - Added __aenter__/__aexit__ methods to Query for cleaner usage patterns.

Changes

• src/claude_agent_sdk/_internal/query.py:

  • Added _reader_cancel_scope for safe cross-task cancellation
  • Added _read_messages_with_cancel_scope() wrapper
  • Updated close() to handle cross-task cleanup gracefully
  • Added async context manager protocol (__aenter__/__aexit__)

• src/claude_agent_sdk/_internal/client.py:

  • Added aclosing() for proper async generator cleanup
  • Added logging and GeneratorExit handling

• tests/test_streaming_client.py:

  • Added TestAsyncGeneratorCleanup test class with 3 tests

Test Plan

• All 134 existing tests pass
• 3 new tests added for async cleanup scenarios
• Ruff linting passes

Example

The issue was reproducible with:

async for message in query(prompt="Say hello", options=options):
    if isinstance(message, ResultMessage):
        print(message.result)
        break  # This would cause RuntimeError

After this fix, breaking out of the loop works cleanly without errors.

Fixes #454