Add three regression tests around the named-pipe transport that
the original PR did not cover. None of these reproduce live bugs
today; they pin invariants the named-pipe path implicitly relies
on so a future change can't silently break them.

TestWriteReadServerInfo_NamedPipe (cross-platform) closes the
producer/consumer loop for the discovery file: an npipe:// URL
written by socketURL must survive WriteServerInfo + ReadServerInfo
without being mangled. The individual emit/parse pieces have
their own tests; this one pins the seam.

TestSetupUnixSocket_NamedPipe_FirstInstanceWins (Windows-only)
asserts that two ListenPipe calls on the same name fail the
second time. winio sets FILE_FLAG_FIRST_PIPE_INSTANCE so that
two thv processes cannot bind the same pipe and race for traffic;
if a future winio bump silently relaxed that, this test catches
it before the discovery layer does in production.

TestCheckHealth_NamedPipe_HungServerCancelsOnContext
(Windows-only) covers the StateUnhealthy path: a peer that
accepts connections but never responds must not wedge
CheckHealth past the caller's context deadline. The existing
success and not-found tests don't exercise this case.

Co-authored-by: Cursor <cursoragent@cursor.com>

TestCheckHealth_NamedPipe_HungServerCancelsOnContext registered
t.Cleanup from inside the Accept loop goroutine. t.Cleanup is
documented to be called from the test goroutine; a registration
that lands after the test body returns can panic with "Log in
goroutine after Test has completed" or be silently dropped. The
test happened to work because the dial CheckHealth makes accepts
before the test body returns, but correctness was load-bearing on
that timing.

Pre-register a single t.Cleanup from the test body that closes
every accepted conn under a mutex, and have the goroutine append
to that slice instead of touching t. Listener cleanup runs in
LIFO order before the conn-close, which unblocks the goroutine's
Accept first so the slice is stable by the time we close it.

Drop the dead "if second != nil { Close }" branch in
TestSetupUnixSocket_NamedPipe_FirstInstanceWins: setupUnixSocket
returns (nil, err) on the named-pipe failure path, so the guard
can never be entered. Replace with a comment so the next reader
does not re-introduce it.

Addresses review on #5216.

Co-authored-by: Cursor <cursoragent@cursor.com>