[DongJinNam]

Description

This PR enhances the spring-ai-mongodb-atlas-store module by:

1. Adding support for ISNULL and ISNOTNULL filter expression types in MongoDBAtlasFilterExpressionConverter.
2. Making the vector index similarity function configurable (COSINE, DOT_PRODUCT, EUCLIDEAN) in MongoDBAtlasVectorStore, instead of being hardcoded to "cosine".

Key Changes

1. MongoDBAtlasFilterExpressionConverter:

   • Implemented ISNULL conversion to {"metadata.<key>": {$eq: null}}.
   • Implemented ISNOTNULL conversion to {"metadata.<key>": {$ne: null}}.
   • Added unit test cases for single and compound/grouped null filter expressions in MongoDBAtlasFilterConverterTest.

2. MongoDBAtlasVectorStore:

   • Introduced MongoDBAtlasVectorStore.SimilarityFunction enum supporting "cosine", "dotProduct", and "euclidean".
   • Added similarityFunction(SimilarityFunction) option to MongoDBAtlasVectorStore.Builder (defaults to SimilarityFunction.COSINE).
   • Updated createSearchIndexDefinition() to use the configured similarity function.
   • Added MongoDBAtlasVectorStoreBuilderTest to verify builder defaults and custom configuration.

Verification

• Tested locally with JDK 25 and Maven Wrapper:
  • MongoDBAtlasFilterConverterTest: 35 tests passed.
  • MongoDBAtlasVectorStoreBuilderTest: 4 tests passed.
  • Checkstyle & spring-javaformat: 0 violations.