Reinstate EnsureIndexAsync implementation with CREATE INDEX logic

[Copilot]

Description

The EnsureIndexAsync method in SqlitePeerStore was reduced to a no-op placeholder, breaking the IPeerStore contract and the existing test EnsureIndexAsync_ShouldCreateIndexWithoutError. This method is expected to create SQLite indexes to optimize queries on document properties.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Changes

Restored Index Creation Logic

• Reinstated full EnsureIndexAsync implementation that creates SQLite indexes using CREATE INDEX IF NOT EXISTS
• Supports both per-collection table mode and legacy single-table mode
• Sanitizes collection names and property paths to prevent SQL injection
• Logs index creation for observability

Security Fix

• Fixed SQL injection vulnerability in legacy mode WHERE clause by using sanitized collection name instead of raw parameter

Compilation Fix

• Fixed GetCollectionsAsync to use _options?.UsePerCollectionTables == true instead of non-existent UsePerCollectionTables property

How Has This Been Tested?

• EnsureIndexAsync_ShouldCreateIndexWithoutError - verifies indexes can be created without errors
• CodeQL security scan - no alerts found

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

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