feat(oss-opensearch): Add Disk-based vector search support

[Akhil-Pathivada]

Overview

Adds Disk-based vector search support for OSS OpenSearch (requires 2.17+).

Features

• Disk-based mode with 6 compression levels (1x, 2x, 4x, 8x, 16x, 32x)
• Auto engine selection based on compression level (Lucene: 1x/4x, FAISS: 2x/8x/16x/32x)
• Rescoring support with configurable oversample factor
• Conditional UI - disk/in-memory options shown contextually
• Version validation - requires OpenSearch 2.17+

Changes

Backend (config.py, oss_opensearch.py)

• Added on_disk, compression_level, oversample_factor configuration
• Implemented CompressionLevel class with engine mapping
• Added resolved_engine property for automatic engine selection
• Extracted _build_vector_field_mapping() for disk vs in-memory modes
• Applied rescoring for both disk-based and in-memory quantization modes

Frontend (models.py, dbCaseConfigs.py)

• Added 3 new enum types for disk-based parameters
• Created 5 OSS-specific UI inputs with conditional display logic
• Reused AWS inputs for common parameters (no duplication)

Technical Notes

• Mutual exclusivity: Disk compression and in-memory quantization cannot coexist
• Backward compatible: Default on_disk=False preserves existing behavior
• No AWS impact: AWS OpenSearch configurations unchanged

Screenshots

[Akhil-Pathivada]

/assign @alwayslove2013

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Akhil-Pathivada, alwayslove2013
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