IGNITE-27926 Implement Raft group destruction in Segstore

[sashapolo]

https://issues.apache.org/jira/browse/IGNITE-27926

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Pull request overview

Implements Raft group destruction support for segstore-backed Raft logs so that per-group log data can be logically tombstoned and later reclaimed by the segment GC.

Changes:

• Add SegmentFileManager.destroyGroup(...) which writes a destroy tombstone via a reset marker.
• Teach IndexFileManager to drop in-memory index metadata for a group when the destroy tombstone is checkpointed.
• Add/extend tests to validate GC behavior and index-meta cleanup after group destroy.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
modules/raft/src/main/java/org/apache/ignite/internal/raft/storage/segstore/SegmentLogStorageManager.java	Switch group destruction to fileManager.destroyGroup(...) instead of reset(...).
modules/raft/src/main/java/org/apache/ignite/internal/raft/storage/segstore/SegmentFileManager.java	Introduce GROUP_DESTROY_LOG_INDEX sentinel and destroyGroup(...) wrapper over reset(...).
modules/raft/src/main/java/org/apache/ignite/internal/raft/storage/segstore/IndexFileManager.java	Remove per-group in-memory index metadata when a destroy sentinel is observed during checkpoint.
modules/raft/src/test/java/org/apache/ignite/internal/raft/storage/segstore/RaftLogGarbageCollectorTest.java	Add GC-focused tests covering segment/index file reclamation after group destruction.
modules/raft/src/test/java/org/apache/ignite/internal/raft/storage/segstore/IndexFileManagerTest.java	Add tests validating that destroy tombstones clear group index metadata on checkpoint.

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[Phillippko]

lgtm