feat: add omnisearch availablity to document Internationalization

[RobbertST]

Description

Added Omnisearch visibility support for the translation.metadata document schema via the metadataOmnisearchVisibility plugin option.

This defaults to true, so metadata documents are still visible from global search unless users explicitly sets with metadataOmnisearchVisibility: false.

This lets users decide whether internal translation metadata documents should appear in Omnisearch.

What to review

Review the new metadataOmnisearchVisibility config option, its default value, and how it is passed into the metadata schema as __experimental_omnisearch_visibility.

Affected areas:

• plugin configuration types
• default plugin config
• translation.metadata schema generation
• README docs
• metadata schema tests

Testing

Added schema tests covering:

• default Omnisearch visibility is true
• configured Omnisearch visibility can be set to false

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🦋 Changeset detected

Latest commit: db3659c

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

This PR includes changesets to release 1 package

Name	Type
@sanity/document-internationalization	Minor

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[bjoerge]

Thanks @RobbertST, nice addition!

Would you mind sorting out this failing test?

[@sanity/document-internationalization] src/schema/translation/metadata.test.ts > metadata schema > sets Omnisearch visibility to false by default
Check failure: [@sanity/document-internationalization] src/schema/translation/metadata.test.ts > metadata schema > sets Omnisearch visibility to false by default
AssertionError: expected true to be false // Object.is equality

- Expected
+ Received

- false
+ true

 ❯ src/schema/translation/metadata.test.ts:105:57

[RobbertST]

Updated tests