SOLR-18329: DenseVectorField support existence queries

[linslee75]

https://issues.apache.org/jira/browse/SOLR-18329

Description

DenseVectorField currently rejects existence queries with BAD_REQUEST.
Lucene has supported vector-field existence checks since 9.1 via
FieldExistsQuery, so Solr should allow the usual existence syntax on
dense vector fields.

Discuss:

• https://lists.apache.org/thread/5rgllrwmxr99k3wwrssbwy0p8nlh3rmx
• https://lists.apache.org/thread/x0x61yh07zzqo73cbct2hdzkybrlgr3m

Solution

• Override DenseVectorField.getExistenceQuery() to return FieldExistsQuery
• Treat unbounded ranges ([* TO *]) as existence queries
• Keep rejecting bounded range queries
• Document existence query support in the dense vector search ref guide
• Add changelog entry

AI coding assistants were used while preparing this change.

Tests

• Updated DenseVectorFieldTest to verify:
  • vector:*
  • -vector:*
  • vector:[* TO *]
• Confirmed bounded range queries still return BAD_REQUEST
• Ran:
  • ./gradlew :solr:core:test --tests org.apache.solr.schema.DenseVectorFieldTest.query_existenceSearch_shouldMatchDocumentsWithVector --tests org.apache.solr.schema.DenseVectorFieldTest.query_rangeSearch_shouldThrowException
  • ./gradlew :solr:core:check -x test

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and this code review
  sheet
• I have created a Jira issue and added the issue ID to this pull request title
• I have given Solr maintainers access to contribute to my PR branch
  (optional but recommended)
• I have developed this patch against the main branch
• I have run ./gradlew check
• I have added tests for my changes
• I have added documentation for the Reference Guide
• I have added a changelog entry

[dsmiley]

Thanks for contributing!