Unify user-defined index record structure with connectors

[qodo-code-review]

Description

• User-defined indexes currently use a different record structure than connectors, causing inconsistency in JavaScript function interfaces
• The EventRecord class lacks exposed schema format information needed by JavaScript selectors
• The old ResolvedEventJsObject implementation needs refactoring to support the unified structure with proper object hierarchy

Deliverables

• Schema Format Exposure: Add SchemaFormat property to EventRecord with lazy initialization from properties
• Record Object Hierarchy: Create RecordObject, PositionObject, and SchemaInfoObject classes to provide unified structure matching connectors
• Property Mapping: Implement MapFrom methods to populate JavaScript objects from ResolvedEvent with sequence ID tracking
• Test Coverage: Update all index tests to use new record properties (rec.schemaInfo.subject, rec.value, rec.headers, etc.)
• Documentation: Update user-defined index documentation with complete record structure specification

[linear]

DB-1801