…ents

A 4.7GB Scylla part copy sent zero response bytes until the copy finished,
so rclone's 5-minute idle timeout killed every attempt (~400s of work vs a
300s deadline) and the proxy kept the doomed copy running as a zombie while
scylla-manager retried forever.

- Commit to 200 OK and trickle whitespace while the copy runs (AWS S3 long-
  copy pattern); emit CopyPartResult or an <Error> document as the body
- Cancel in-flight backend work when the client disconnects (no zombies)
- Run passthrough segment copies concurrently (bounded, default 8) instead
  of ~570 sequential backend calls
- Overlap the synthetic-ETag MD5 source pass with the segment copies
  instead of serially re-reading the whole source after them
- Fix ruff check failures (F841, B039, import order) so CI lint passes

…ion failures)

test_copy_full_encrypted_object: the single-segment passthrough copy
returned a synthetic plaintext etag to the client but stored it as the
part etag too, so CompleteMultipartUpload forwarded an etag the backend
never issued -> InvalidPart. This was masked before ae39244: the proxy
turned the backend error into a 500, boto3 retried, and the retry
succeeded via state reconstruction (which reads real etags from
list_parts). Store the backend CopyPartResult etag in part state and
have _build_s3_parts use stored etags instead of the client echo.

test_part_number_out_of_range: rejecting UploadPart before reading its
body (new internal-part-range validation) left the body bytes on the
keep-alive connection, so the client's next request (the abort) hit a
raw uvicorn 400. Drain small bodies in the error handler; send
Connection: close for bodies too large to slurp.

Both reproduced locally against real MinIO and verified fixed.

…manifests

When part 1 hybrid passthrough ends with a sub-5MB mid-frame tail and part 2
follows, store the tail in upload state instead of uploading it as a non-final
internal S3 part. Part 2 prepends the deferred bytes into streaming; complete
flushes any leftover tail. Adds debug logs and regression tests.

Co-authored-by: Cursor <cursoragent@cursor.com>

Mark 500+ segment Scylla prod-shape unit tests as slow, exclude them
from parallel test-unit, and run them with -n0 in a separate CI step.

Co-authored-by: Cursor <cursoragent@cursor.com>

Streaming copy now calls take_deferred_copy_tail at part start; test
_Mgr/_TrackingMgr stubs were missing the method.

Co-authored-by: Cursor <cursoragent@cursor.com>

Move prod-scale unit tests out of the parallel unit job into unit-slow
with a longer timeout. Shrink part 2 in the two-part prod-shape test so
it still exercises streaming + deferred tail without ~1.3GB of work.

Co-authored-by: Cursor <cursoragent@cursor.com>

572-segment part 1 + part 2 streaming on a ~5GB mock source exhausts
runner memory. Coverage is split: prod part-1 passthrough (slow) and
two-part complete with deferred tail (fast).

Co-authored-by: Cursor <cursoragent@cursor.com>