Move the write-and-close helpers from S3FileSystem to S3File #1034

Problem

S3FileSystem has two static helpers that write to a file opened for writing and close it without committing on failure:

• S3FileSystem._write_and_close(f, value), added for pipe_file() in Write pipe_file() data without committing a failed write #1003.
• S3FileSystem._write_file_and_close(f, local, callback), added for put_file() in Leave the existing object unchanged when put_file() fails #1017.

Neither uses the filesystem. Both operate only on the S3File they receive, and call its private _close_without_commit() from outside the class. AioS3FileSystem does not inherit S3FileSystem, so it reaches them through its internal filesystem (self._sync_fs._write_and_close(...), self._sync_fs._write_file_and_close(...)), although AioS3File inherits S3File.

Proposed change

Move both helpers to S3File as instance methods, next to _close_without_commit(), and call them on the opened file:

self.open(path, "wb", **kwargs)._write_and_close(value)
self.open(rpath, "wb", ...)._write_file_and_close(local, callback)

Private methods only; no behavior change.