fix(redis): loop SCAN until keys found or cursor reaches zero to prevent empty result lockup

[ZhuchkaTriplesix]

Summary

In RedisKeysView._scanBatch(), SCAN cursor MATCH <pattern> COUNT 100 was previously executed only once per batch. When scanning large keyspaces with a filter pattern, Redis can return 0 matching keys alongside a non-zero cursor. Previously, when 0 keys were returned on initial load, _keys.isEmpty caused the view to render the "No keys found" placeholder and completely hid the ListView, hiding the "Load more" button and permanently locking the user out of the keyspace.

Changes

• In RedisKeysView._scanBatch(), loop scan calls until at least some keys are discovered, cursor reaches 0, or a safety iteration limit is reached.
• In RedisKeysView.build(), if _keys.isEmpty but _hasMore == true (cursor > 0), display a visible "Continue scanning" action in the empty state instead of permanently locking the UI.
• Added comprehensive unit tests in test/features/redis/redis_keys_view_test.dart verifying empty-first-batch continuation and the "Continue scanning" flow.

Closes #920