feat: rich context typing for SliceZone component

[wuguishifu]

Resolves:
N/A

Description

This PR introduces the ability to richly type a <SliceZone /> component, allowing for strict type checking to ensure that context parameters exist downstream in slices.

Right now, there's no way of statically guaranteeing that properties exist in the context which means you need to do additional defensive programming in each slice.

This change allows static analysis to catch any errors with missing context params much earlier in the dev flow.

Note: This feature will not break existing repos as the generic type is not required on the SliceZone component. Anyone who upgrades to a version of @prismicio/react with these changes will not have any type issues (this can be seen in ./examples/with-typescript/index.tsx:94

Checklist

• If my changes require tests, I added them.
• If my changes affect backward compatibility, it has been discussed.
• If my changes require an update to the CONTRIBUTING.md guide, I updated it.

Preview

How to QA ¹

This is a type-only change, so running npm run types should be enough to validate it.

🤓

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Note

Low Risk
Compile-time-only API change with a default generic that preserves existing optional context usage.

Overview
SliceZone can now take a generic context type so slice components get a typed context prop and TypeScript can catch missing or wrong-shaped context at compile time.

SliceZoneProps uses a conditional type: context stays optional when no generic is used (same as today), but context is required when you pass something like SliceZone<RichContext>. The component is implemented as a generic function (SliceZone<T = never>) instead of FC<SliceZoneProps>, with no runtime behavior change.

The TypeScript example adds WithContextRichSliceZone to show the required context object when a context contract is declared.

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