Add typed builder API for vector search index definitions

[rozza]

• Introduced SearchIndexDefinition sealed interface with factory methods,
• VectorSearchIndexFields (vectorField, filterField, autoEmbedField builders),
• HnswSearchIndexOptions for HNSW-specific parameters, and
• VectorSearchIndexDefinition as the concrete implementation. Updates
• Updated SearchIndexModel javadoc to reference the new builders.

JAVA-6112

[Copilot]

Pull request overview

This PR introduces a typed Java builder API for creating Atlas Vector Search index definitions (and supporting options), and wires the new public types into the Scala driver surface area. It also adds unit tests validating the BSON produced by the new builders.

Changes:

• Added SearchIndexDefinition with vectorSearch(...) factory methods and a concrete VectorSearchIndexDefinition implementation.
• Added VectorSearchIndexFields builders (vector/filter/auto-embed fields) and HnswSearchIndexOptions for HNSW parameters.
• Updated SearchIndexModel Javadoc and added unit tests covering common definition shapes and BSON output.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
driver-scala/src/main/scala/org/mongodb/scala/model/package.scala	Exposes new Java model types to Scala via type aliases.
driver-core/src/test/unit/com/mongodb/client/model/SearchIndexDefinitionTest.java	Adds unit tests asserting BSON output for the new vector index definition builders.
driver-core/src/main/com/mongodb/client/model/VectorSearchIndexFields.java	Introduces fluent builders for vector search index field definitions.
driver-core/src/main/com/mongodb/client/model/VectorSearchIndexDefinition.java	Implements a SearchIndexDefinition producing { fields: [...] }.
driver-core/src/main/com/mongodb/client/model/SearchIndexModel.java	Updates Javadoc to reference the new vector index definition builders.
driver-core/src/main/com/mongodb/client/model/SearchIndexDefinition.java	Adds a sealed interface + factories for typed search index definitions (currently vector search).
driver-core/src/main/com/mongodb/client/model/HnswSearchIndexOptions.java	Adds a small fluent builder for HNSW-specific options, implementing Bson.

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