Add more async lifecycle tests and fix potential POSIX cancellation fd leak

[puddly]

This PR adds an explicit test suite for the asyncio.Protocol state machine pattern and aligns platform behavior around poorly-timed cancellation. These aren't things normally hit at runtime often but it's not good practice to leak file descriptors, especially when we use flock.

We also plug a few more cancellation point leaks. This pattern is a bit annoying to deal with because any await point can potentially be cancelled but with the asterisk that we are often awaiting on the result of a thread pool executor, which cannot be cancelled. This requires a bit of coordination with future callbacks and such.

For Windows, we now intentionally detach the file handle from PyHANDLE and let it "leak" for consistency with POSIX, since the cleanup code should work without a magical __del__.

[codecov]

Codecov Report

❌ Patch coverage is 93.84615% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.21%. Comparing base (b068974) to head (96c29c3).

Files with missing lines	Patch %	Lines
serialx/platforms/serial_rfc2217/__init__.py	81.81%	2 Missing ⚠️
serialx/platforms/serial_win32.py	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #93      +/-   ##
==========================================
+ Coverage   91.46%   92.21%   +0.74%     
==========================================
  Files          22       22              
  Lines        3599     3621      +22     
==========================================
+ Hits         3292     3339      +47     
+ Misses        307      282      -25     

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[Copilot]

Pull request overview

Hardens async serial transport lifecycle: introduces shared state-tracking in BaseSerialTransport (made/lost/user-close flags), shields the open executor future on POSIX and Windows so a cancellation cannot leak the fd/HANDLE, detaches the Windows PyHANDLE to surface (rather than mask) missing explicit closes, and adds a comprehensive asyncio.Protocol lifecycle test suite plus a hub4com stdout drainer to keep its pipes from filling up.

Changes:

• Centralize connection_made_called / connection_lost_called / user_initiated_close in BaseSerialTransport and use asyncio.shield around the open executor future to plug cancellation-race resource leaks (POSIX fd, Windows HANDLE via new CreateFile_detached).
• Make per-backend transports (serial_posix, serial_win32, serial_socket, serial_rfc2217, serial_esphome, serial_pyodide) route through the new helpers and respect the user-initiated-close flag to avoid spurious "broken" reports.
• Add tests/test_async_lifecycle.py covering normal close, abort, cancellation during connect, drain vs abort, re-entrant close in connection_made, fd-leak race, etc.; drain hub4com stdout/stderr in tests to prevent stalls.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
serialx/common.py	Adds shared lifecycle flags and idempotent _call_protocol_connection_made/_lost helpers.
serialx/descriptor_transport.py	Shields the os.open executor future and delegates close/abort/connection-lost to shared helpers.
serialx/platforms/serial_posix.py	Switches to shared _call_protocol_connection_made helper.
serialx/platforms/serial_win32.py	Adds CreateFile_detached, shields executor future, adds _connection_made_waiter, uses shared helpers.
serialx/platforms/serial_socket.py	Uses shared flags; only marks broken on non-user-initiated close.
serialx/platforms/serial_rfc2217/init.py	Uses shared flags; failures of pending waiters still produce some exception.
serialx/platforms/serial_esphome.py	Uses shared helpers; drops writes after close; notes TODO for sync force-disconnect.
serialx/platforms/serial_pyodide/init.py	Drops writes after close; clears write queue on abort.
tests/common.py	Drains hub4com stdout/stderr via threads to avoid pipe buffer stalls.
tests/test_async_lifecycle.py	New extensive Protocol-state lifecycle test suite incl. cancellation race.

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