aws_s3: close the object scanner when reading an object fails

[a-palamarchuk]

Fixes #2617.

What

When the aws_s3 input's scanner returned an error other than io.EOF while
reading an object, ReadBatch cleared a.object and returned without closing the
scanner. Because a.object was already cleared, Close couldn't reach it later
either. The scanner, and the S3 GetObject response body (plus any decompression
reader) it wraps, leaked for every object that failed mid-read, holding a
connection from the HTTP client's pool.

This closes the scanner on every read error, not only at end of object. The close
result goes into its own variable, so a close failure is logged and can't replace
the original read error returned to the caller.

Is it safe to close after an error?

Yes. Closing doesn't turn the failure into a successful acknowledgement of the S3
object, which in SQS mode would delete the notification:

• Scanners built on service.AutoAggregateBatchScannerAcks (every built-in Benthos
  and Connect scanner) acknowledge the source with the read error inside
  NextBatch, through a once-only guard. Their Close then only releases the
  reader; the later acknowledgements are no-ops.
• The deprecated codec: readers in Benthos follow the same pattern.
• The wrapper scanners (decompress, skip_bom, switch) pass the source
  acknowledgement straight to the scanner they wrap.

So the object is still reported as failed exactly as before; the only change is
that its resources are released.

Tests

• New TestReadBatchClosesScannerOnError plants a scanner that fails on read and
  records whether it was closed. It fails on main (scanner never closed) and
  passes with this change, and also checks the original error is still returned.
• go test -race -shuffle=on ./internal/impl/aws/s3/... passes.
• TestIntegrationS3 (LocalStack) passes, including the SQS line-reading subtests
  for both the current scanner and the deprecated codec: path, which exercise the
  normal end-of-object close that this change moves.
• golangci-lint run and golangci-lint fmt --diff are clean.