serial: uart_native_pty: IRQ support breaks pytest shell interaction

[henrikbrixandersen]

Describe the bug

serial: uart_native_pty: IRQ support introduced in #93957 breaks pytest native_sim shell interaction in odd ways. Interaction is very, very slow compared to before, and spurious characters are transmitted.

As an example, see #94424 (comment) with an example output from running tests/drivers/can/host:

DEBUG    twister_harness.helpers.shell:shell.py:49 Got prompt
DEBUG    twister_harness.device.device_adapter:device_adapter.py:150 #: uart:~$ ean show can
DEBUG    twister_harness.device.device_adapter:device_adapter.py:150 #: ean: command not found

The pytest code never writes ean show can, it writes can show can.

Regression

• This is a regression.

Steps to reproduce

I am reproducing this with a script from https://github.com/cANnectivity/cannectivity/ but it's a bit of a complicated setup. If you want, you can follow https://docs.zephyrproject.org/latest/develop/manifest/external/cannectivity.html to pull in CANnectivity and then run the https://github.com/CANnectivity/cannectivity/blob/main/scripts/run-host-tests.sh script.

Relevant log output

Impact

Showstopper – Prevents release or major functionality; system unusable.

Environment

• OS: Ubuntu Linux
• Zephyr SHA: 9eb8e0c
• Zephyr SDK 0.17.2

Additional Context

No response