Show all STRINGS choices in cache editor dropdown

[Quan Zhuo (quanzhuo)]

This change addresses item #5022

This changes visible behavior

The following changes are proposed:

• Replace the Cache Editor's native datalist control for STRINGS cache variables with a custom editable dropdown.
• Show all declared STRINGS choices whenever the dropdown is opened, instead of filtering choices by the current input value.
• Preserve manual text editing and invalid-choice highlighting for string cache entries.
• Add a regression test for rendering STRINGS choices as an unfiltered popup list.

[Quan Zhuo (quanzhuo)]

The native datalist in VS Code webviews filters options based on the current input value and cannot be configured to always show all CMake STRINGS choices, so this replaces it with a small editable combobox implementation.

[Quan Zhuo (quanzhuo)]

One behavior choice worth calling out: cmake-gui shows STRINGS cache entries as a non-editable dropdown. This PR keeps CMake Tools' existing editable input behavior and only fixes the filtered candidate list. I’m happy to adjust this to a non-editable dropdown if maintainers prefer matching cmake-gui exactly.