feat: add OSS OpenSearch and semantic FTS support

[jamesgao-jpg]

Summary

This PR adds OSS OpenSearch and the backend support needed for both unfiltered and filtered full-text search benchmarking. Filtered FTS frontend presentation and generated result publication remain deferred.

PR #794 introduced the preceding full-text search benchmark foundation used by this work. This PR supersedes #814 with a reduced publication scope. PR #818 introduces shared CLI run-context notes; its commit is incorporated here so these FTS runs can preserve deployment and constraint context in result JSONs.

Changes

• add the OSS OpenSearch backend client, configuration, CLI registration, and focused tests
• retain unfiltered semantic FTS metrics and controls
• restore filtered FTS backend cases, dataset preparation, runner orchestration, metrics, and CLI --fts-filter-rate control
• distribute FTS filter_id values with a deterministic bijection so exact-selectivity filters are scattered across corpus insertion order rather than selecting a contiguous suffix
• expose --fts-filter-id-distribution [permuted|sequential], defaulting to permuted, and propagate the selected mode through case construction, dataset preparation, qrel filtering, and result metadata
• restore filtered FTS request translation for Milvus, Elastic Cloud, OSS OpenSearch, TurboPuffer, and Vespa
• keep the standard FTS QPS, recall, NDCG, MRR, and load frontend views without exposing filtered FTS cases
• report OSS OpenSearch bulk item failures with the actual successful-document count instead of treating partial writes as full success
• add shared --note and --note-file CLI options while preserving Pinecone compatibility
• preserve current upstream backend and compatibility changes by incorporating the latest main

Deferred Scope

• filtered FTS frontend controls and presentation
• generated filtered FTS result JSON artifacts
• session and documentation-only migration files

The filtered FTS backend and CLI paths are included, but filtered results are not published in the frontend by this PR.

Compatibility

FTS collections loaded before commit bf85cf7 contain sequential filter_id values. Reload those collections before running filtered FTS search so stored IDs and semantic qrel filtering use the same mapping. Changing --fts-filter-id-distribution between permuted and sequential also requires reloading the collection.

Verification

• make lint
• make unittest
• 75 focused filtered-FTS backend, case, CLI, dataset, runner, OpenSearch, and TurboPuffer tests
• DCO verification for every outgoing commit
• VDBBench pre-push gate and impact-map validation
• exact production restoration recorded in commit 659f739, followed by isolated current-lint compatibility commit a352b73
• deterministic exact-selectivity FTS filter distribution added in commit bf85cf7
• explicit sequential/permuted CLI and case switch added in commit e24110b

No live provider probe was run for this update; provider request translation is covered by mocked unit tests.

[sre-ci-robot]

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