Fix #562: default task missing positionals raises ParseError

[DSeaStar]

Summary

• When a collection has a default task and no task name is given, Executor.normalize used to call that task with no arguments. Required positionals then failed inside the task body as a TypeError traceback (custom Program binaries, or inv with no argv).
• The default-task path now checks the same required-positional rules as the parser and raises ParseError, so Program.run prints the familiar 'task' did not receive required positional arguments: 'arg' message and exits 1.
• Adds unit/CLI tests for the missing-arg case and for a default task that only has optional arguments.

Fixes #562.

Test plan

• Executor.execute() with a default task missing a required positional raises ParseError (not TypeError)
• Program.run("myapp") with that collection prints the parser-style error on stderr
• Default task with only optional args still runs
• Existing default_tasks_called_when_no_tasks_specified (Mock body / *args, **kwargs) still passes
• Named-task missing positionals still uses the original parser error path