The "server is closing" error produced when a connection shuts down
because its write side failed formatted the underlying write error with
`%v`, so the cause - typically `io.EOF` when a stdio host closes its
pipe - was not part of the error chain. A caller that wants to
distinguish a clean host disconnect from a real failure via
`errors.Is(err, io.EOF)` therefore always got `false`.
This changes both `s.writeErr` wrappings to use `%w`, preserving the
write error in the error chain while keeping the same message text
("server is closing: EOF").
A regression test exercises the write-failure branch of `shuttingDown`
and asserts both `errors.Is(err, ErrServerClosing)` and `errors.Is(err,
io.EOF)`.
Fixes#1098
---------
Co-authored-by: Bo Zhang <187063395+Tethys0@users.noreply.github.com>
Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>
