shell: fix unsafe API calls and add configurable autoflush behavior

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.

Link: https://github.com/zephyrproject-rtos/zephyr/issues/84274



Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@verkada.com>