### What problem does this PR solve?
This PR fixes the merge-phase crash reported in #14236 during GraphRAG
entity resolution.
The issue happens after candidate pair resolution completes, when
multiple merge coroutines mutate the same shared `networkx` graph
concurrently. In `_merge_graph_nodes`, the code iterates over
`graph.neighbors(node1)` and also awaits during edge/description
merging. That allows another coroutine to modify the graph adjacency
structure in between, which can trigger `RuntimeError: dictionary keys
changed during iteration` and can also lead to unsafe shared-graph
mutation.
This change keeps the PR scoped to that single issue by:
- serializing merge-time graph mutations with a dedicated merge lock
- snapshotting `graph.neighbors(node1)` with `list(...)` before
iteration
Together, these changes prevent concurrent mutation of the shared graph
during the merge phase and make the merge loop safe against live-view
invalidation.
Fixes#14236
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
GraphRAG _async_chat.
### Type of change
- [x] Refactoring
- [x] Performance Improvement
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **Refactor**
* Unified chat calls to an async invocation across extractors, improving
timeout handling and ensuring task IDs propagate reliably.
* **Tests**
* Added and expanded unit tests and mocks to cover extractor behavior,
timeout scenarios, and safe test-package imports, reducing regression
risk.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
