Fix ABOR while waiting for a data connection

[charan-rathore]

When ABOR arrives after a transfer command but before the data socket connects, the readiness decorator receives the cancellation outside the worker's abort handler. The server closes the control connection instead of acknowledging the abort.

Move the worker decorator outside the data-connection check for LIST, MLSD, STOR (including APPE), and RETR. Cancellation during the wait now sends the existing 426/226 responses. Also cancel and drain the readiness gather when its caller is cancelled, shielding each connection future so cancellation does not break subsequent transfers.

The regression tests use a real TCP server on IPv4 and IPv6. They check both response codes and a successful download on the same control connection after the abort. A timeout control checks that the existing 425 response and later transfer still work.

Closes #205.

Validation:

• CPython 3.12.14: all 10 new abort cases fail on the unmodified server (ConnectionResetError); patched full suite: 804 passed.
• Fresh-clone run_tests.sh on CPython 3.10.12: 10 baseline failures; patched suite: 798 passed, 1 skipped. Version-dependent test parametrization produces different totals.
• Ruff check and format check passed on both runs; mypy passed.
• Existing tests cover active-transfer aborts and no-work ABOR.

[greptile-apps]

Confidence Score: 5/5

[Medium risk] Fixes abort handling during data connection setup.

No outstanding findings block merging.

_{Reviews (3) · Last reviewed commit: "Give the abort test replies a 1 second d..."}

[charan-rathore]

Closing for now since nothing has moved here. Happy to pick it back up if a maintainer wants it.