[v17] MB-27666: Nested Fields

[CascadingRadium]

• Introduce NestedSegment interface to manage hierarchical document relationships.
• Provide methods to get ancestor IDs, count root documents, and handle nested deletions.
• Enables consistent handling of nested documents across segments.

[Copilot]

Pull Request Overview

This PR introduces support for nested document relationships by adding a NestedSegment interface that extends the existing Segment interface with hierarchical functionality.

• Adds NestedSegment interface with Ancestors and Descendants methods
• Provides optional backward compatibility for segments that don't support nested documents
• Enables retrieval of parent-child document relationships within segments

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