Add MUVERA processors for multi-vector search

[praveenMprasad]

Description

Add MUVERA (Multi-Vector Retrieval via Fixed Dimensional Encodings) ingest and search request processors to enable ANN prefetch for multi-vector models like ColBERT and ColPali.

The MUVERA algorithm converts variable-length multi-vector embeddings into a single fixed-dimensional encoding (FDE) vector using SimHash clustering and random projections. This enables approximate nearest neighbor search on multi-vector representations, which previously required brute-force scoring via lateInteractionScore.

Two new processors:

• muvera ingest processor — reads multi-vectors from a source field, produces an FDE vector, writes it to a knn_vector target field. Original multi-vectors remain in _source for reranking.
• muvera_query search request processor — intercepts script_score queries containing query_vectors in script params, MUVERA-encodes them, and replaces the inner match_all with a knn query on the FDE field. The lateInteractionScore script wrapper stays intact for MaxSim reranking.

Key design decisions:

• All hyperparameters have sensible defaults (k_sim=4, dim_proj=8, r_reps=20, seed=42)
• FDE dimension is computed and logged at pipeline creation time
• Optional fde_dimension parameter validates against computed value
• Tested end-to-end on a live cluster with correct MaxSim reranking scores

Reference: MUVERA paper, fastembed implementation

Related Issues

Resolves #3163

Check List

• New functionality includes testing.
  • 34 unit tests across 3 test classes, stable across 10 iterations with random seeds
• New functionality has been documented.
  • Documentation PR to follow after initial review
• API changes companion pull request created.
  • No REST API changes — processors are configured via existing ingest/search pipeline APIs
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.
  • Will create after initial review

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[navneet1v]

@praveenMprasad please add the IT for this new Processor.

[codecov]

Codecov Report

❌ Patch coverage is 86.45418% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.59%. Comparing base (5419381) to head (eb2b695).

Files with missing lines	Patch %	Lines
...processor/muvera/MuveraSearchRequestProcessor.java	78.94%	8 Missing and 12 partials ⚠️
...ch/knn/processor/muvera/MuveraIngestProcessor.java	83.78%	7 Missing and 5 partials ⚠️
...opensearch/knn/processor/muvera/MuveraEncoder.java	97.50%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3164      +/-   ##
============================================
+ Coverage     82.52%   82.59%   +0.06%     
- Complexity     3935     4001      +66     
============================================
  Files           426      429       +3     
  Lines         14646    14897     +251     
  Branches       1869     1935      +66     
============================================
+ Hits          12087    12304     +217     
- Misses         1796     1811      +15     
- Partials        763      782      +19     

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[praveenMprasad]

@praveenMprasad please add the IT for this new Processor.

@navneet1v Added MuveraProcessorIT with 7 integration tests covering the end-to-end flow - ingest pipeline creation, FDE encoding on indexed documents, search pipeline with ANN prefetch via muvera_query, passthrough for non-script_score queries, and error handling for dimension mismatches and invalid fde_dimension config. Please take a look.

[navneet1v]

Thank you @praveenMprasad . Will start reviewing the PR. I would need some to review it, since I had to first understand what MUVERA is.

[navneet1v]

@praveenMprasad before we can kickoff the review lets ensure that all CIs are passing.

cc: @Gankris96 and @kotwanikunal

[praveenMprasad]

@navneet1v I've fixed the compilation issue. The CI workflows need maintainer approval to run, could you please approve them ?