feat(index): introduce "isolated" option

[Haroenv]

An isolated index does not get requested when changes happen in the parent and also does not inherit its search parameters.

If you want an isolated index to search, you need to cause a search (eg by refining a child widget)

isolated indices:

• do not automatically search when mounted
• do not show up in ui state
• do not interact with onStateChange
• do not get requested in server-side rendering
• do not cause searches in parent indices

This option is EXPERIMENTAL, and implementation details may change in the future. Things that could change are:

• which widgets get rendered when a change happens
• whether the index searches automatically
• whether the index is included in the URL / UiState
• whether the index is include in server-side rendering

Usage:

search.addWidgets([
  index({ EXPERIMENTAL_isolated: true, indexName: 'alone' })
]);

search.addWidgets([
  index({ EXPERIMENTAL_isolated: false }).addWidgets([
    index({ indexName: 'isolated' })
  ]),
]);

FX-3448

supersedes the following POCs:

• closes #6880
• closes poc: have indices that send independent requests #5939

[Copilot]

Pull Request Overview

This PR introduces an experimental "isolated" option for the index widget that allows indices to operate independently from their parent context. Isolated indices do not automatically search when mounted, are excluded from UI state, don't interact with onStateChange, and don't get requested in server-side rendering.

Key changes:

• Modified IndexWidgetParams type to support optional isolated configuration with union types
• Implemented isolation logic affecting search behavior, parent relationships, and UI state inclusion
• Added comprehensive test coverage for isolated index functionality

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
packages/instantsearch.js/src/widgets/index/index.ts	Core implementation of isolated index functionality including type definitions, search behavior modifications, and parent relationship handling
packages/instantsearch.js/src/widgets/index/tests/index-test.ts	Comprehensive test suite covering isolated index behavior, UI state exclusion, and search triggering scenarios
packages/instantsearch.js/src/lib/InstantSearch.ts	Updated composition client validation to exclude isolated indices from the restriction

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Sandbox	Source
example-instantsearch-getting-started	Configuration
example-react-instantsearch-getting-started	Configuration
example-react-instantsearch-next-app-dir-example	Configuration
example-react-instantsearch-next-routing-example	Configuration
example-vue-instantsearch-getting-started	Configuration

[dhayab]

Awesome addition 👏