debugger: handle read errors on POSIX pipe and test abrupt disconnect

The POSIX pipe read wrappers stored the return of ::read() into an
unsigned result, so a negative count (e.g. EBADF from a concurrent
close on another thread) became SIZE_MAX. ContentReader::buffer()
would then try to grow its deque by ~18 exabytes and crash with
either std::length_error (glibc) or a stack smash (libc++). Treat
any non-positive ::read() return as EOF/error, close the pipe, and
return 0 so the peer's SessionThread observes a clean empty payload.

cmDebuggerPipeClient is test-only infrastructure; production cmake
is always the pipe server. Production close() assumes sequential
access, which holds in normal use (the adapter destructor joins
SessionThread before closing the connection). The new abrupt-
disconnect test however needs to wake a sibling thread blocked in
read() on the same fd, which Linux ::close() does not do.

Add a ShutdownForTesting() method that calls shutdown(SHUT_RDWR)
without freeing the fd, so any concurrent blocking read wakes
with a clean zero-length return. On Windows, CloseHandle already
cancels pending overlapped I/O, so the helper just forwards to
close(). Use it from testProtocolWithPipesAbruptDisconnect in
place of close().