fix: Allow to tag non-exhaustive union types

[acote-coveo]

Description

Add a new x-non-exhaustive OpenAPI vendor extension that marks discriminated union (tagged) types as non-exhaustive. Union types are considered "non-exhaustive" when they can be extended through plugins at runtime — code generators should treat these unions as open/extensible rather than sealed.

Changes

• New vendor extension: Introduced x-non-exhaustive: true annotation on the following plugin-extensible union schemas:

  • Property (_common.mapping.yaml)
  • Analyzer, TokenizerDefinition, TokenFilterDefinition, CharFilterDefinition (_common.analysis.yaml)
  • QueryContainer, SpanQuery (_common.query_dsl.yaml)
  • AggregationContainer (_common.aggregations.yaml)
  • ProcessorContainer (ingest._common.yaml)
  • FieldSuggester (_core.search.yaml)

• Linter/Tester updates: Registered x-non-exhaustive as a recognized additional keyword in both SchemasValidator.ts and SchemaValidator.ts to prevent unknown-keyword lint errors.

• Documentation: Added x-non-exhaustive to the vendor extensions section of DEVELOPER_GUIDE.md.

• Changelog: Documented the addition under the "Added" section.

Motivation

Generators (e.g. opensearch-java) need a way to know which union types are open to extension so they can emit appropriate code — such as an "custom" variant or a custom deserialization fallback — rather than failing on unrecognized discriminator values introduced by plugins.

Related Issues

• [opensearch-project/opensearch-java#2083](Feat/add support for non exhaustive union types opensearch-java#2083)

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