Demonstrates add_msg pollution bug across dialogs

Setup: Create two dialogs, 'dlg_a' and 'dlg_b'

In 'dlg_a', run this code in the same message:

msg1 = add_msg("Message in dlg_b", dname='dlg_b', placement='at_end')
print(f"Created {msg1} in dlg_b")
print(f"__msg_id is now: {__msg_id}")

# Now add a message to this dialog (dlg_a)

msg2 = add_msg("Message in same dialog")
print(f"Created {msg2} - should be in dlg_a, after msg1")

Bug:

msg2 doesn't get created anywhere because __msg_id in 'dlg_a' scope was set by the first add_msg call with a message id from dlg_b.

Impact: Any function that calls add_msg with different dname values pollutes the caller's __msg_id, breaking subsequent message placement. If the add_msg calls happen in different messages, __msg_id is reset to the message id and the issue won't happen.

Possible fix: add_msg should call set_var('__msg_id', ...) only if dname parameter is not set.

Workaround: Always pass explicit id and placement parameters when calling several add_msg after cross-dialog operations in one message.