Add AcceptsPositionalArgs regression tests for all non-positional clets

[Copilot]

The AcceptsPositionalArgs property and validation logic (D-025, PR #55) already landed on develop. However, only IntCletTests had an explicit AcceptsPositionalArgs_IsFalse regression test among non-positional clets.

This PR adds the missing regression tests for all 12 remaining non-positional clets:

• ColorCletTests
• ConfirmCletTests
• DateCletTests
• DecimalCletTests
• DurationCletTests
• TimeCletTests
• RangeCletTests
• AttributePickerCletTests
• TextCletTests
• MultilineTextCletTests
• PickFileCletTests
• PickDirectoryCletTests

Each test follows the established pattern:

[Fact]
public void AcceptsPositionalArgs_IsFalse ()
{
    IClet clet = new ColorClet ();

    Assert.False (clet.AcceptsPositionalArgs);
}

Tests via the IClet interface to exercise the default interface member. Catches accidental => true overrides on clets that should reject positional args.