close_async() returns early without calling _close_components() or transport.kill() when used with a sync transport. This means if someone uses async with client: context manager with a sync transport, neither the session flusher, batchers, monitor, nor the transport itself will be properly cleaned up. The sync close() should be called as a fallback to ensure resources are released.

Also found at:

• tests/integrations/asyncio/test_asyncio.py:665-673

The change adds common: httpcore[asyncio] as a dependency for all common test environments. However, httpcore 1.x (which is specified in setup.py via httpcore[asyncio]==1.*) requires Python >=3.8. The common environment includes Python 3.6 and 3.7 configurations (line 21: {py3.6,py3.7,...}-common), which will cause pip installation failures when tox tries to install this dependency on those Python versions.

The make_transport function has multiple fallback paths when async transport is requested but cannot be used: (1) no event loop running, (2) AsyncioIntegration not enabled, (3) httpcore[asyncio] not installed. None of these fallback paths have explicit test coverage to verify the warning messages are logged and sync transport is correctly used instead. This could lead to regressions in the fallback behavior going unnoticed.