fix(sftp): pass original position to read() callback instead of internal cursor

[jeffrson]

Fixes #1488

Problem

sftp.read(handle, buf, off, len, position, cb) passes the wrong value as the position argument to cb when len exceeds _maxReadLen. The internal field req.position is used both as a mutable cursor for sequential sub-reads and as the return value for the user callback — so the callback always receives the end-offset of the last sub-read instead of the original position.

Fix

Introduce req.origPosition to preserve the original value across sub-reads, analogous to the existing req.origOff pattern for the off argument.

Diff

     const req = (req_ || {
       nb: 0,
       position,
  +    origPosition: position,
       off,
       origOff: off,
       ...
       cb: (err, data, nb) => {
         ...
  -      cb(undefined, req.nb + nb, data, req.position);
  +      cb(undefined, req.nb + nb, data, req.origPosition);
       },
     });

Testing

• Triggers reliably with any non-OpenSSH server and a read request > 31952 bytes
• With OpenSSH servers the same bug occurs for requests > 260096 bytes (~254 KB)

Prior art

This was previously reported as part of #1171 (attached to a different bug). The PR also contains a test case demonstrating the failure.