Validate subparser class and remove default subcommand title

[kmvanbrunt]

• Improved attach_subcommand() by validating that attached parsers are
  Cmd2ArgumentParser instances and match the subparsers group's
  parser_class.

• Removed default 'subcommands' title from add_subparsers() to align
  with standard argparse behavior.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.51%. Comparing base (b2b888c) to head (156d158).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1636   +/-   ##
=======================================
  Coverage   99.51%   99.51%           
=======================================
  Files          21       21           
  Lines        4699     4699           
=======================================
  Hits         4676     4676           
  Misses         23       23           

Flag	Coverage Δ
unittests	99.51% <100.00%> (ø)

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