Refactor `MappingRedisOMConverter` into focused converter components

[sagile]

MappingRedisOMConverter has grown large and now owns several distinct responsibilities: entity read/write orchestration, hash bucket path handling, map/collection reconstruction, type hint handling, custom conversion, and low-level nested value logic. Recent nested map/collection fixes added more complexity to an already dense class.

This issue tracks a dedicated refactor-only PR to reduce the size and improve maintainability without changing behavior.

Suggested scope:

• Extract nested map/collection hash read/write logic into focused helper classes.
• Keep public behavior and serialized Redis hash format unchanged.
• Preserve existing converter APIs and Spring Data integration points.
• Add or retain targeted regression coverage around nested maps, collections, type hints, and simple scalar values.
• Avoid unrelated formatting or behavior changes.

Success criteria:

• Existing full test suite remains green.
• MappingRedisOMConverter is smaller and easier to navigate.
• Extracted code has clear ownership boundaries and no public API churn.