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
Type-only API tightening with backward-compatible defaults; no runtime logic changes beyond the existing context default.

Overview
Adds rich context typing for <SliceZone /> via a generic TContext, so slice components can rely on statically checked context instead of defensive checks at runtime.

SliceZoneProps now uses a conditional type: context stays optional when the default unknown context is used (existing call sites unchanged), but context is required when you pass a concrete type like <SliceZone<RichContext>>. The component is implemented as a generic function (SliceZone<T = never>) instead of React.FC, with the same runtime behavior (context still defaults to {}).

The TypeScript example adds WithContextRichSliceZone to document the pattern.

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