Keep a pack-observer Scanner error from aborting the upload

The observer tees the pack stream through a goroutine running packfile.Scanner. When that Scanner stopped early (a malformed pack, or content it couldn't parse), its deferred pr.Close() broke the pipe, so the next TeeReader write failed with io.ErrClosedPipe and surfaced from Read — aborting the live push, despite the documented "non-fatal for the upload" contract.

Tee through a bestEffortWriter that absorbs write failures and always reports success, so a stopped observer can never turn into a failed upload. The bytes keep flowing to the server and the Scanner error remains available via ScannerError() for debugging. (This removes the spurious push failure, so there is no longer an unclassified error for executeBatched to mishandle.)