feat(symfony-console): implement the shell completion command plumbing

The _complete and completion commands were registered but always panicked:
get_class_of_command / instantiate_completion_output / tail_debug_log were
todo!() and the completion.bash resource was not shipped.

- make Command::complete return anyhow::Result so completion errors
  propagate to CompleteCommand's catch-all (exit code 2) like PHP
- add Command::get_class as the port hook for PHP's get_class() debug log;
  every command supplies its PHP FQCN via the delegation macro
- embed Resources/completion.bash at compile time (single-binary port);
  get_supported_shells becomes a static list
- implement tail_debug_log by moving the shared output handle into the
  'static process callback
- add OutputInterface::as_console_output so unsupported-shell errors go to
  stderr as in PHP
- fix CompletionInput::bind to keep the argument name PHP assigns in the
  foreach head even when the loop breaks on the first unset argument;
  application-level completion always hit this and returned no suggestions

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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