gh-154467: Fix empty (Pdb) prompt when attaching to a process with pdb -p (#154469)

gh-154467: Fix empty (Pdb) prompt when attaching to a process with pdb -p (#154469)

_PdbServer inherits _cmdloop, which wraps cmdloop() in
_maybe_use_pyrepl_as_stdin(). That context manager blanks self.prompt to ''
so that a local pyrepl draws the prompt itself. The remote server, however,
never reads from a local pyrepl -- it transmits self.prompt to the client
over the socket -- so the blanking made it send an empty prompt whenever the
target process had a pyrepl-capable terminal (pyrepl_input is set).

Override _maybe_use_pyrepl_as_stdin() in _PdbServer to a no-op, keeping the
real prompt. Add an integration test that attaches to a target running under
a pty, so PyREPL is genuinely enabled inside it, and asserts the transmitted
prompt is "(Pdb) ".

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>