fix: stabilize graph reranking and community propagation

[RitwijParmar]

Summary

• fix node episode-mentions reranking so higher mention counts rank first while zero-mention nodes stay last
• make community label propagation deterministic and bounded so repeated entity edges cannot oscillate forever
• add pure unit coverage for positive mention-count ordering and repeated-edge community propagation

Validation

• uv run --extra falkordb --with pytest --with pytest-asyncio pytest tests/utils/search/search_utils_test.py tests/utils/maintenance/test_community_operations.py -q
• uv run --with ruff ruff check graphiti_core/search/search_utils.py graphiti_core/utils/maintenance/community_operations.py tests/utils/search/search_utils_test.py tests/utils/maintenance/test_community_operations.py
• uv run --with ruff ruff format --check graphiti_core/search/search_utils.py graphiti_core/utils/maintenance/community_operations.py tests/utils/search/search_utils_test.py tests/utils/maintenance/test_community_operations.py
• git diff --check

Fixes #1342
Addresses #1355

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[RitwijParmar]

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