Add --dry-run flag to pipx ensurepath

[somaz94]

Adds a --dry-run flag to pipx ensurepath that reports which directories
would be added to PATH without modifying PATH or any shell configuration
file, so users can preview the change before running it for real (closes #1014).

The flag threads a dry_run parameter through ensure_pipx_paths →
ensure_path; in dry-run mode userpath.append/prepend are never called and
a "Would append/prepend ..." line plus a "no changes were made" footer are
printed instead.

• ran the linter to address style issues (pre-commit run --all-files)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in changelog.d/ folder (feature)
• updated/extended the documentation

Validation

• New tests/test_ensurepath.py (4 tests — first test file for this command):
  asserts dry-run never calls userpath.append/prepend, never mutates config,
  and checks the footer + EXIT_CODE_OK.
• ruff check + ruff format --check clean on all changed files.
• End-to-end pipx ensurepath --dry-run --force prints Would append ... and
  the no-changes footer without touching PATH.