[Backport v4.0-branch] shell: fix unsafe API calls and add configurable autoflush behavior

[zephyrbot]

Backport b0a0feb~2..b0a0feb from #84290.

Fixes #84274

Introduced a new Kconfig option SHELL_PRINTF_AUTOFLUSH to allow
configuring the autoflush behavior of shell printing functions.

Updated Z_SHELL_FPRINTF_DEFINE to use the
CONFIG_SHELL_PRINTF_AUTOFLUSH setting instead of hardcoding
the autoflush behavior to true.

Fixes an issue where the shell API could block indefinitely when called
from threads other than the shell's processing thread, especially when
the transport (e.g. USB CDC ACM) was unavailable or inactive.

Replaced k_mutex_lock calls with an indefinite timeout (K_FOREVER)
by using a fixed timeout (K_MSEC(SHELL_TX_MTX_TIMEOUT_MS)) in shell
API functions to prevent indefinite blocking.

[jakub-uC]

@carlescufi @kartben

I am a bit confused why this PR is failing:

ERROR:root:https://github.com/zephyrproject-rtos/zephyr/pull/84440 does not have an associated issue
ERROR:root:Please ensure the body of each PR to a release branch contains "Fixes #1234"
ERROR:root:The following PRs are lacking associated issues:
ERROR:root:https://github.com/zephyrproject-rtos/zephyr/pull/84440

while at the same it is forbidden to use the fixes keyword as it is a reference to the zephyr github repository.

Is it an inconsistency or are backport branches special?

[pdgendt]

@carlescufi @kartben

I am a bit confused why this PR is failing:

ERROR:root:https://github.com/zephyrproject-rtos/zephyr/pull/84440 does not have an associated issue
ERROR:root:Please ensure the body of each PR to a release branch contains "Fixes #1234"
ERROR:root:The following PRs are lacking associated issues:
ERROR:root:https://github.com/zephyrproject-rtos/zephyr/pull/84440

while at the same it is forbidden to use the fixes keyword as it is a reference to the zephyr github repository.

Is it an inconsistency or are backport branches special?

I've updated the PR description, should be fixed once the bot sees the "Fixes ..." in the PR description.

[pdgendt]

while at the same it is forbidden to use the fixes keyword as it is a reference to the zephyr github repository.

Commits should not reference PRs or issues, but PR descriptions can and should.