feat: add OSS OpenSearch and semantic FTS support

[jamesgao-jpg]

Summary

This PR adds the backend and frontend support needed for unfiltered full-text search benchmarking while deferring publication of filtered FTS results.

PR #794 introduced the preceding full-text search benchmark foundation used by this work. This PR supersedes #814 with a reduced scope that excludes filtered FTS frontend and result support.

Changes

• add the OSS OpenSearch backend client, configuration, CLI registration, and focused tests
• retain unfiltered semantic FTS metrics and controls
• retain the standard FTS QPS, recall, NDCG, MRR, and load frontend views
• remove filtered FTS datasets, cases, runners, backend paths, CLI options, tests, result artifacts, and frontend views
• preserve current upstream backend and compatibility changes by incorporating the latest main

Deferred Scope

Filtered FTS search results and their frontend presentation are intentionally deferred and are not published by this PR.

Verification

• make lint
• make unittest
• 32 focused FTS/OpenSearch tests
• DCO verification for every commit in upstream/main..fts_v2_backend
• VDBBench pre-push gate and impact-map validation
• confirmed that no filtered FTS symbols or filtered result artifacts remain in the outgoing diff

[sre-ci-robot]

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