feat(redis): folder tree view for keys by delimiter

[ZhuchkaTriplesix]

Closes #1025

Changes

• Implemented RedisKeyTreeEngine in lib/features/redis/redis_key_tree_engine.dart to partition and build hierarchical trees of folders and leaf keys by configurable delimiter (:, /, ., or custom).
• Added Flat / Tree View mode toggle in RedisKeysView toolbar using material.SegmentedButton.
• Added delimiter input field with instant re-partitioning and expand all / collapse all buttons.
• Rendered virtual folders with total key counts, chevron animation, and delete folder action using confirmDestructiveAction (DestructiveSqlType.redisDel).
• Added delMany in RedisConnection and supported multiple keys in RedisConnectionTestFake.
• Added comprehensive unit tests in redis_key_tree_engine_test.dart and widget tests in redis_keys_view_test.dart.