SOLR-18267: Add scalar quantized flat vector index support with no HNSW

[adamjq]

https://issues.apache.org/jira/browse/SOLR-18267

Description

This PR follows up from #4492, which added a new knnAlgorithm=flat option to DenseVectorField. It adds the same flat index support to ScalarQuantizedDenseVectorField so that scalar-quantized fields can also be indexed without building HNSW graphs.

The new behavior introduced is:

• ScalarQuantizedDenseVectorField now supports knnAlgorithm=flat. Searching works through the {!knn}, {!knn_text_to_vector}, and {!vectorSimilarity} query parsers, and the vectorSimilarity() function query.
• ScalarQuantizedDenseVectorField with knnAlgorithm=flat and vectorEncoding=BYTE is rejected explicitly

Note: knnAlgorithm=flat is not supported for BinaryQuantizedDenseVectorField

Solution

This PR adds support for ScalarQuantizedDenseVectorField with knnAlgorithm=flat by delegating to Lucene104ScalarQuantizedVectorsFormat.

This implementation differs from #4492 in two main ways:

1. The reader in Lucene104ScalarQuantizedVectorsFormat implements brute-force KNN search, meaning the knn, knn_text_to_vector and vectorSimilarity query parsers are supported by default
2. Lucene104ScalarQuantizedVectorsFormat is registered in SPI, so no wrapper class is needed

The table below shows the different segment files generated between the flat vs HNSW configurations:

Ext	Contents	Scalar Quantized (Flat)	Scalar Quantized (HNSW)
.vec	Raw float vectors	Yes	Yes
.vemf	Raw-float metadata	Yes	Yes
.veq	Quantized vectors	Yes	Yes
.vemq	Quantized metadata	Yes	Yes
.vem	HNSW graph metadata	No	Yes
.vex	HNSW graph	No	Yes

AI Disclosure: Claude was used to assist with this PR. All code has been reviewed and tested by me.

Tests

Unit tests for ScalarQuantizedDenseVectorField.

Checklist

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• I have developed this patch against the main branch.
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