fix(cancel): wait for the attention ack across message boundaries

[datlechin]

Fixes #459.

• flush_done_attention reads past end-of-message until a DONE, DONEPROC or DONEINPROC with DONE_ATTN arrives, and discards everything before it, ERROR tokens included.
• try_unfold keeps its behavior. It now calls a private unfold(stop_at_end_of_message), which the attention drain calls with false.
• A closed transport still fails the cancel, with UnexpectedEof. command_timeout still bounds each read.
• Unit tests in tds::stream::token: the ack in the next message, the next request reading its own response after a cancel, and transport EOF before the ack.
• Integration test cancel_query_after_dropping_a_running_query (tokio and smol): drops a running WAITFOR DELAY '00:00:30' after 300 ms, cancels, then runs SELECT 1.

Against mcr.microsoft.com/mssql/server:2022-CU14-ubuntu-22.04:

• before: cancel_query returns Err(Protocol("Never got a DONE token acknowledging the Attention signal.")) and the next SELECT 1 returns no row
• after: cancel_query returns Ok in 1 to 2 ms and SELECT 1 returns 1