Refa: GraphRAG to use async chat methods instead of thread pool execution (#14002)

### 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 -->

rag/graphrag/entity_resolution.py

@@ -32,7 +32,6 @@ from rag.graphrag.utils import perform_variable_replacements, chat_limiter, Grap
 from api.db.services.task_service import has_canceled
 from common.exceptions import TaskCanceledException
 
-from common.misc_utils import thread_pool_exec
 
 DEFAULT_RECORD_DELIMITER = "##"
 DEFAULT_ENTITY_INDEX_DELIMITER = "<|>"
@@ -213,21 +212,15 @@ class EntityResolution(Extractor):
             timeout_seconds = 280 if os.environ.get("ENABLE_TIMEOUT_ASSERTION") else 1000000000
             try:
                 response = await asyncio.wait_for(
-                    thread_pool_exec(
-                        self._chat,
-                        text,
-                        [{"role": "user", "content": "Output:"}],
-                        {},
-                        task_id
-                    ),
+                    self._async_chat(text, [{"role": "user", "content": "Output:"}], {}, task_id),
                     timeout=timeout_seconds,
                 )
 
             except asyncio.TimeoutError:
-                logging.warning("_resolve_candidate._chat timeout, skipping...")
+                logging.warning("_resolve_candidate._async_chat timeout, skipping...")
                 return
             except Exception as e:
-                logging.error(f"_resolve_candidate._chat failed: {e}")
+                logging.error(f"_resolve_candidate._async_chat failed: {e}")
                 return
 
         logging.debug(f"_resolve_candidate chat prompt: {text}\nchat response: {response}")