Add iter()/aiter() for filter-based lazy iteration

Summary

Add lazy filter-based ID/key iteration APIs for index maintenance and agent pipelines.

Level: Intermediate

Current State

• paginate() yields full document records, not IDs/keys only.
• scan_by_pattern() works on raw Redis key patterns and returns a materialized list.
• No index filter-expression based generator for IDs/keys.

Proposed Change

• Add lazy iterators over keys included in an index in Redis optionally applying some filtering:
  • iter(filter_expression...)
  • aiter(filter_expression,...)
• Yield as a generator of said keys?

Definition of Done

• Iteration is lazy (streaming batches, no full list required).
• Works for selective and wildcard filter expressions.
• Integration tests verify complete and deterministic coverage.

Out of Scope

• Arbitrary cursor checkpoints or resumable iterators.

Read the existing paginate() and scan_by_pattern() implementations and their integration tests first, then trace the index filter-expression paths for both sync and async APIs. Done means selective and wildcard expressions lazily stream complete, deterministic key coverage without materializing the full result, with integration tests verifying both iterators.