Add ContainsOption and TrySelectOption to option widgets

[TomW-Skyline]

Introduce ContainsOption and TrySelectOption methods to dropdown, radio button, and checkbox list widgets (generic and non-generic). These methods allow checking for the existence of options and safely attempting selection. Updated interfaces, implementations, and XML docs to improve API consistency and usability.

AB#41138

[ThomasRemmery]

@copilot can you review the PR?

[Copilot]

@copilot can you review the PR?

Here's the review summary with the issues found and fixed (commit 728caff):

Issues fixed:

1. Incomplete XML docs on TrySelectOption — Added <param> and <returns> tags to all TrySelectOption method signatures in IDropDown and IRadioButtonList. The ContainsOption methods already had them; TrySelectOption was missing them.

2. Inconsistent null checks for generic T value parameters — ContainsOption(T value) and TrySelectOption(T value) in the generic implementations threw ArgumentNullException for null, but the existing RemoveOption(T value) doesn't. This is also problematic for value types where value == null is unreachable. Removed to align with the rest of the codebase.

3. Missing unit tests — No tests existed for the new methods. Added tests covering exists/not-exists cases and null-argument guard cases across all affected widget types (DropDownTests, CheckBoxListTests, additions to GenericDropDownTests, GenericRadioButtonListTests, GenericCheckBoxListTests, RadioButtonListTests).

No other issues found. Implementation is correct: ContainsOption properly delegates to the underlying collection, TrySelectOption correctly reuses ContainsOption before assigning, and the Option<T> overloads still retain null guards since Option<T> is a reference type.