fix(NumberField): add labelledBy prop for aria-labelledby on spinbutton

[sridhar-3009]

Summary

• NumberField.Control only supports accessible naming via the label string prop (rendered as aria-label). Consumers who render a visible <label> element outside the component cannot wire it to the spinbutton, and omitting label silently produces a spinbutton with no accessible name — a WCAG 4.1.2 failure
• Adds labelledBy string prop to NumberField.Root that flows through context to NumberField.Control and renders as aria-labelledby on the spinbutton
• When labelledBy is set, aria-label is suppressed to avoid redundant or conflicting accessible names

What changed

• NumberFieldRoot.vue: labelledBy?: string added to NumberFieldRootProps and NumberFieldRootContext; destructured in defineProps and threaded through context
• NumberFieldControl.vue: aria-label is now conditional on labelledBy being absent; aria-labelledby set from root.labelledBy when present
• index.test.ts: three new tests — aria-labelledby set from prop, aria-label suppressed when labelledBy is present, aria-label absent when neither prop is provided

Test plan

• labelledBy="my-label" → aria-labelledby="my-label" on the spinbutton
• label="Quantity" labelledBy="my-label" → aria-labelledby wins, no aria-label
• No props → no aria-label on spinbutton
• All 138 NumberField tests pass

Fixes a WCAG 4.1.2 gap where consumers with visible external labels had no way to wire them to the spinbutton.

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