SOLR-18074 DenseVectorField multi-valued fields / block-join (BACKPORT)

[dsmiley]

https://issues.apache.org/jira/browse/SOLR-18074

back-port to branch_9x
Implemented by GitHub Copilot. It did absolutely everything, including some tricky parts... amazing!

What follows are Copilot's summary:

Cherry-picks commit 949b28e for SOLR-18074 onto branch_9x, enabling multi-valued DenseVectorField support by transparently converting multi-valued vector fields into Lucene block-join child documents at index time.

Changes

Cherry-pick (949b28e) — resolved conflicts

• NestedUpdateProcessorFactory: Detects multi-valued vector fields at root level and converts each value into a child nested doc under a pseudo-field (NESTED_VECTORS_PSEUDO_FIELD_NAME), removing the original field
• ChildDocTransformer: Reconstructs multi-valued vectors from nested children back onto the parent doc in search results
• DenseVectorField / IndexSchema: Minor supporting changes for multi-valued awareness
• schema-densevector.xml: Added nested doc fields (_root_, _nest_path_), multi-valued vector fields, and dynamic fields for new tests
• Ref guide: Documents multi-valued dense vector indexing syntax and behavior

branch_9x compatibility fixes

• Java 11: Replaced instanceof pattern matching (Java 16+) with explicit cast in NestedUpdateProcessorFactory

• KnnQParser: Ported childrenOf and parents.preFilter parameters from main — required by the new tests but absent on this branch. Uses DiversifyingChildrenFloatKnnVectorQuery / DiversifyingChildrenByteKnnVectorQuery from Lucene (already present in Lucene 9.12.3):

  {!knn f=vector_multivalued topK=3 childrenOf=$allParents parents.preFilter=$someParents}[1,1,1,1]
  

• ChildDocTransformer: Fixed byte vector extraction to respect BytesRef.offset and BytesRef.length (the original iterated the entire backing array)

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