Treat read() returning no bytes as a permanent failure

[puddly]

Serial ports on POSIX enter a strange state on device disconnect: select will return that n bytes can be read but read(n) returns no bytes. We should enter an instance-wide failure state if this is a case (not just on read) because writes and pin state changes should fail.

Related: home-assistant/core#170257

[codecov]

Codecov Report

❌ Patch coverage is 97.75281% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.61%. Comparing base (c8be924) to head (8b88c11).
⚠️ Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
serialx/platforms/serial_rfc2217/__init__.py	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #87      +/-   ##
==========================================
+ Coverage   90.86%   91.61%   +0.75%     
==========================================
  Files          22       22              
  Lines        3482     3603     +121     
==========================================
+ Hits         3164     3301     +137     
+ Misses        318      302      -16     

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[Copilot]

Pull request overview

This PR hardens disconnect handling by treating “readable but read() returns no bytes” (common on POSIX after device unplug) as a persistent, instance-wide failure. This prevents consumers from silently receiving EOF (b"") and then continuing to write or query/set modem pins against a broken connection.

Changes:

• Add a persistent “broken” state to serial instances/transports and check it across reads, writes, and modem-pin operations.
• Treat zero-byte reads as a disconnect on POSIX (sync and asyncio fd-based transports), closing/errored appropriately instead of EOFing.
• Extend test infrastructure with an “unplug” capability (socat) and add new tests asserting that operations raise after unplug.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/test_sync_transports.py	Adds sync unplug regression test ensuring operations raise after disconnect.
tests/test_async_transports.py	Adds async unplug regression tests (AsyncSerial + StreamReader) expecting exceptions after disconnect.
tests/conftest.py	Threads unplug callables through the serial_pair fixture.
tests/common.py	Adds NO_UNPLUG quirk, extends SerialPair, and upgrades create_socat_pair() to provide unplug callables.
tests/test_serial_linux.py	Updates create_socat_pair() tuple unpacking after signature change.
tests/test_serial_esphome.py	Updates create_socat_pair() tuple unpacking after signature change.
serialx/common.py	Introduces persistent broken-state tracking and enforces it on read/write/modem-pin APIs (sync + transport-side checks).
serialx/platforms/serial_posix.py	Marks serial instance broken when a low-level read returns 0 bytes and raises EIO.
serialx/descriptor_transport.py	Converts zero-byte fd reads into a broken-state + errored close (prevents busy-looping on b"").
serialx/platforms/serial_rfc2217/init.py	Marks RFC2217 as broken on connection close and on TCP connection lost.

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