Move z_shell_backend_rx_buffer_flush() before state_set() in
shell_start() to prevent a race condition where incoming shell
commands could be lost.
Previously, the sequence was:
1. state_set() - prints the prompt
2. z_shell_backend_rx_buffer_flush() - flushes RX buffer
If the shell thread was preempted after printing the prompt,
the host could see the prompt and send commands. When the
thread resumed, z_shell_backend_rx_buffer_flush() would discard
those commands.
By flushing the RX buffer before printing the prompt, any
commands received after the prompt is visible will not be
affected by the flush operation.
Fixes#99674
Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@verkada.com>
a04d8957c2