Move AioS3StreamedFile writes onto the multipart write session #1228

Sub-issue of #1211, which records the design decisions. Depends on the session added for S3File.

Use case

AioS3StreamedFile duplicates the part tracking of S3File with its own differences: _discard() only logs an abort failure, _committing guards a queued commit, and __del__ decides from implicit state whether to abort.

Proposed change

Move AioS3StreamedFile onto _S3WriteSession:

• Writes await the session's readiness with asyncio.wrap_future; a cancelled task cancels only its own readiness future.
• The commit reserves completion with begin_complete() before run_shielded() queues it, replacing _committing; the abort and __del__ use the session state.
• The single-request path, abort-failure handling, and state errors match S3File.

Behavior changes are listed in the PR for the 4.2.0 release notes.

Validation plan (if implementing)

• The existing AioS3StreamedFile tests, including cancellation, loop shutdown, garbage collection, and one-worker pools.
• Tests that a cancellation while the commit is queued does not abort it, and that failed part futures and bodies are released after the error is consumed.
• The filesystem integration tests against the CI bucket.