fix(plan): accept empty input as default, fix duplicate -p, add test and warnings

[zph]

• When user presses Enter at a parameter prompt, treat empty input as
  use default by parsing default in value_proc (fixes JSONDecodeError).
• Change --prompter short option from -p to -r to avoid conflict with --params.
• Use datetime.now(timezone.utc) instead of deprecated utcnow().
• Add regression test that plans with newlines to accept all defaults.
• Derive prompt count from notebook params so test is robust.
• Filter pytest deprecation warnings from jupyter_client and papermill.