fix: preserve non-nullable ref return types

[dvd233]

🤔 This is a ...

• New feature
• Bug fix
• Site / documentation update
• Demo update
• TypeScript definition update
• Bundle size optimization
• Performance optimization
• Enhancement feature
• Internationalization
• Refactoring
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🔗 Related issue link

Fixes #2808

💡 Background and solution

After the React 19 type upgrade, the inferred declarations for useLatest and useUnmountedRef changed from their documented mutable, non-nullable ref contracts to RefObject. Because ahooks still supports React 16–19 and older React type packages define RefObject<T>.current as T | null, consumers can see a nullable current even though both hooks initialize it synchronously and never introduce null themselves.

This change explicitly preserves the public contracts as MutableRefObject<T> and MutableRefObject<boolean>, whose current stays non-nullable across the supported React type versions. It also adds declaration-emission regression tests alongside the existing runtime coverage. No runtime behavior changes.

Validation:

• vitest run packages/hooks/src/__tests__/refTypes.spec.ts packages/hooks/src/useLatest/__tests__/index.spec.ts packages/hooks/src/useUnmountedRef/__tests__/index.spec.ts --no-file-parallelism --maxWorkers=1 (3 files, 5 tests passed)
• vitest run --color (93 files, 406 tests passed)
• tsc --noEmit -p packages/hooks/tsconfig.json
• declaration-only emit verified both public signatures

📝 Changelog

Language	Changelog
🇺🇸 English	Restore non-nullable mutable ref return types for useLatest and useUnmountedRef.
🇨🇳 Chinese	恢复 useLatest 和 useUnmountedRef 非空可变 ref 的返回类型。

☑️ Self Check before Merge

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• TypeScript definition is updated/provided or not needed
• Changelog is provided or not needed

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1 out of 2 committers have signed the CLA.

✅ dvd233
❌ crazylxr
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[crazylxr]

LGTM

[pkg-pr-new]

npm i https://pkg.pr.new/ahooks@2956

npm i https://pkg.pr.new/@ahooks.js/use-url-state@2956

commit: 560bcb9