fix(tasks): add hover description for required property in taskDefinitions contribution schema (#275670)

[maruthang]

What — Add markdownDescription to the required property in the taskDefinitions extension-point schema.

Why — Hovering the property in an extension's package.json previously showed only the schema type with no documentation (#275670).

How — Added a localized nls.localize('TaskDefinition.required', ...) matching the style of the sibling when property's markdownDescription.

Test plan — Create an extension with a contributes.taskDefinitions entry including a required array and hover over the required key in package.json — the hover card now shows the description. npm run compile-check-ts-native passes cleanly.

Fixes #275670