[No QA] refactor: OptionRowLHN — DescriptiveText subcomponent

[mountiny]

Part of

#89946 — OptionRowLHN compound component decomposition

Step

10 of 12 — depends on #89947 (Provider + context interface). Incorporates work from #89753.

What

Extract OptionRow.DescriptiveText into src/components/LHNOptionsList/DescriptiveText.tsx.

Contains:

• optionItem.descriptiveText display
• Fullstory fsClass (alternateTextFSClass) application
• Renders nothing when descriptiveText is falsy
• Reads all required data from RowContext

Why

Descriptive text is a conditionally-rendered label distinct from title and subtitle. This also carries forward the intent of #89753.

Acceptance criteria

• descriptiveText rendering and its null-guard live only in DescriptiveText.tsx
• No visible behavior or style changes

Issue Owner

Current Issue Owner: @DylanDylann