Normalize key prefixes by removing trailing separators when constructing
  Redis keys to prevent double separator issues (e.g., "user::123" becomes
  "user:123").

  Changes:
  - Fix BaseStorage._key() to strip trailing separators from prefix
  - Update SemanticRouter to use index's key_separator instead of hardcoded ':'
  - Extract route pattern generation into reusable _route_pattern() method
  - Fix scan pattern generation to respect custom separators