Add optional query bit size hint to KnnSearchStrategy.Hnsw

[arup-chauhan]

This PR introduces an optional query bit-size hint for KnnSearchStrategy.Hnsw as a first incremental step toward #15614.

I intentionally limited the scope to search-strategy API plumbing and tests to keep risk low.
Default behavior remains unchanged: the new hint is metadata-only in this PR and does not alter scoring yet.

Changes

• Added optional queryBitSizeHint to KnnSearchStrategy.Hnsw.
• Kept backward compatibility by preserving the existing constructor and adding an overload:
  • Hnsw(int filteredSearchThreshold)
  • Hnsw(int filteredSearchThreshold, Integer queryBitSizeHint)
• Added validation for the hint (> 0 when non-null).
• Added getter: queryBitSizeHint().
• Updated equals / hashCode for Hnsw to include the new hint.
• Updated KnnSearchStrategy.Patience constructors to support hint passthrough.
• Updated HnswQueueSaturationCollector#getSearchStrategy() to preserve and forward the hint when wrapping strategies.
• Added new tests in TestKnnSearchStrategy to cover:
  • default/no-hint behavior
  • constructor with hint
  • hint validation
  • equals/hashCode behavior
  • hint preservation across seeded/patience wrapping paths

Validation

• ./gradlew -p lucene/core compileJava
• ./gradlew -p lucene/core test --tests TestKnnSearchStrategy
• ./gradlew -p lucene/core check

Only tweak I made: removed “(after formatting fix)” since it passes now.

[arup-chauhan]

Hey @benwtrent

Implemented the first incremental step by adding an optional queryBitSizeHint to KnnSearchStrategy.Hnsw.

I’ve preserved backward compatibility, forwarded the hint through Patience / seeded wrapping paths, and added focused strategy tests. This PR intentionally does not change scoring behavior.

Next, I plan to wire the hint into the vector reader/scorer plumbing, and then follow up with a small PR that uses the hint for query-bit-aware behavior in the scalar-quantized scoring path.

Looking forward to your feedback!