fix: make fnllm utils fork-safe for Celery workers

[droideronline]

Detect process forks and recreate thread resources to prevent 'RuntimeError: threads can only be started once' in worker processes.

• Add PID tracking for fork detection
• Safe cleanup of inherited resources
• Fresh event loop creation per process

Problem

GraphRAG fails in Celery worker processes with RuntimeError: threads can only be started once when executing queries that use embedding models. This occurs because the current run_coroutine_sync() implementation is not fork-safe.

Root Cause

When Celery creates worker processes by forking:

1. Child processes inherit parent's global thread objects (_thr, _loop, _pid)
2. Inherited thread objects exist but the actual threads are not running (threads don't survive forks)
3. Code attempts to call _thr.start() on a dead thread object
4. Python raises "threads can only be started once" because threads have single-use lifecycle

Solution

This PR implements fork detection and safe resource recreation:

• Fork Detection: Track process ID to detect when code runs in a forked child process
• Safe Cleanup: Use contextlib.suppress() to safely cleanup inherited resources
• Fresh Resources: Create new event loop and thread specific to the child process
• Process Isolation: Each forked process gets its own async infrastructure

Changes Made

Modified Files:

• graphrag/language_model/providers/fnllm/utils.py

Related Issues

#1974

Proposed Changes

1. Added imports: os and contextlib for PID tracking and safe cleanup
2. Enhanced run_coroutine_sync(): Added fork detection logic
3. Process tracking: Compare current PID with stored PID to detect forks
4. Resource cleanup: Safely stop inherited event loops before creating new ones
5. Thread recreation: Create fresh thread and event loop for each process

Checklist

• I have tested these changes locally.
• I have reviewed the code changes.
• I have updated the documentation (if necessary).
• I have added appropriate unit tests (if applicable).

[droideronline]

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[droideronline]

@natoverse - Could you please review this PR when you have some time. thanks