feat(oss-opensearch): Add KNN derived source configuration option

[Akhil-Pathivada]

Problem

OpenSearch 3.x+ enables knn.derived_source.enabled by default, which allows OpenSearch to derive vectors from the vector values file instead of storing them in the _source field. However, users need a way to control this behavior for their specific use cases:

• When working with different OpenSearch versions because this feature was not be available (< 3.x)
• When they want to disable this feature to store vectors in _source for specific requirements

Solution

Added a new configurable option knn_derived_source_enabled allowing users to:

• "None" (default) - Skip setting for OpenSearch < 3.x compatibility
• "True" - Explicitly enable (3.x+ default behavior)
• "False" - Disable for better performance on 3.x+

Changes

• Added knn_derived_source_enabled: str | None = None field to OSSOpenSearchIndexConfig
• Conditional setting in _create_index() - only added when explicitly set
• UI dropdown with 3 options and version guidance
• Added enum to CaseConfigParamType

Benefits

• ✅ Version-compatible (OpenSearch < 3.x and 3.x+)
• ✅ User-friendly UI with clear guidance
• ✅ No breaking changes - default behavior unchanged

Files Changed

• vectordb_bench/backend/clients/oss_opensearch/config.py
• vectordb_bench/models.py
• vectordb_bench/backend/clients/oss_opensearch/oss_opensearch.py
• vectordb_bench/frontend/config/dbCaseConfigs.py

Screenshots

[sre-ci-robot]

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[Akhil-Pathivada]

/assign @alwayslove2013

[Akhil-Pathivada]

Hi @alwayslove2013 I've opened a foundational PR #635, which introduces a new version-aware system for OpenSearch settings.

I'm converting this PR to a draft as it depends on the foundational system in #635. Once that merges, I'll update this branch with the new design and mark it as ready for review.