Handle missing root communities in dynamic selection

[Yin Li (yinli-systems)]

Summary

Fixes #2382 by allowing DynamicCommunitySelection to initialize when the community hierarchy has no level 0 entries. In that case there are no root communities to seed the dynamic traversal, so the selector now starts with an empty queue instead of raising KeyError during construction.

Tests

• git diff --check
• PYTHONPYCACHEPREFIX=/private/tmp/graphrag-pycache /usr/bin/python3 -m py_compile packages/graphrag/graphrag/query/context_builder/dynamic_community_selection.py tests/unit/query/context_builder/dynamic_community_selection.py
• UV_CACHE_DIR=/private/tmp/uv-cache-graphrag uv run ruff check packages/graphrag/graphrag/query/context_builder/dynamic_community_selection.py tests/unit/query/context_builder/dynamic_community_selection.py
• UV_CACHE_DIR=/private/tmp/uv-cache-graphrag uv run ruff format --check packages/graphrag/graphrag/query/context_builder/dynamic_community_selection.py tests/unit/query/context_builder/dynamic_community_selection.py
• UV_CACHE_DIR=/private/tmp/uv-cache-graphrag PYTHONPATH=packages/graphrag uv run --package graphrag pytest -q tests/unit/query/context_builder/dynamic_community_selection.py

[Yin Li (yinli-systems)]

Closing after re-auditing the maintainer guidance on #2382 and the earlier #2384. Treating a missing level-0 community as an empty traversal would mask an indexing invariant violation, which Nathan Evans (@natoverse) explicitly identified as the more serious underlying problem. This PR therefore should not merge in its current direction.