Fix PropsMerge type so setNodes merge can return scalars

[cpruijsen]

description

Allow PropsMerge callbacks to return scalar property values.
packages/slate/src/interfaces/editor.ts changes the return type from object to unknown.
setNodes calls merge for each property and stores the result in Record<string, unknown>.

issue

Refs #5490.
Property-specific callback parameter types remain open, so the issue's explicitly annotated examples still fail strict function checks.

example

For a custom text node with a key property:

Transforms.setNodes(
  editor,
  { key: 'value' },
  {
    at,
    merge(existingValue, newValue) {
      if (newValue === 'value') {
        return newValue
      }
      return existingValue
    },
  }
)

Before the change, TypeScript rejects this callback because unknown is not assignable to object. The new declaration accepts the callback.

context

The shared declaration covers Transforms.setNodes and editor.setNodes and keeps the current unknown parameter contract.
Property-specific parameter and return inference must account for absent existing properties and multiple supplied keys.

packages/slate/test/props-merge.ts compiles snippets against Slate source with the existing TypeScript dependency because the Mocha Babel loader removes TypeScript annotations.

checks

• The changed files pass ESLint and Prettier.
• Slate source passes the TypeScript check without emit.
• Tests pass with yarn test --watchman=false --runInBand: 1,423 Mocha tests and 97 Jest tests.
• The regression file passes alone and fails against the merge-base source with the expected return-type assertion.
• The change includes a patch changeset.
• Full repository yarn lint.
• Interactive examples and integration tests.

Tests use Node 22.17.0, which matches CI.

[changeset-bot]

🦋 Changeset detected

Latest commit: 0a92d70

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
slate	Patch

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[dylans]

• since Fix and Type Tests (and some type errors I found along the way) #6003, packages/slate/test is part of tsc --build, so yarn lint:typescript already type-checks test files in CI
• a plain typed call in a test file covers this, no compiler harness needed
• e.g. Transforms.setNodes(editor, {}, { merge: (prop: unknown, node: unknown) => node }) fails on main and passes with this change
• this harness also type-checks all of slate's source every time mocha loads
• rebase onto main (19 commits behind, no conflicts), then swap this file for the typed call

[cpruijsen]

Rebased onto main and swapped the harness for a typed setNodes call. yarn lint:typescript fails with TS2322 on the old PropsMerge and passes with this change.