feat: store collection relation cache in Redis when enabled- #1305

[romitshah02]

Adds a Redis-backed path for the collection hierarchy relationship-rebuild API, part of a larger cross-repo change (knowlg-publish job now writes hierarchy data to Redis on publish; lern-service /content/state/update now reads from Redis) — this repo's piece covers the "rebuild if missing" API.

POST /collection/v4/hierarchy/update/relation/:identifier → RelationManager.updateHierarchyRelationships computes leaf-node, optional-node, and ancestor-node maps from a collection's hierarchy and persists them. Previously always wrote to the Yugabyte-backed hierarchy_relations Cassandra table via ExternalStore. Now, when redis.enable=true, it writes each map as a simple key → node-id-list entry in Redis via the existing RedisCache.saveList, keyed as $rootId:$identifier:$relationshipType (same key shape as before). Falls back to the existing Yugabyte path when Redis is disabled — no schema/config changes required, no new dependency (module already pulls in platform-cache transitively via graph-engine, same as HierarchyManager.scala in this module).

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)
• This change requires a documentation update

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
• Any dependent changes have been merged and published in downstream modules

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