Fix race condition in MainThreadRunner with thread-indexed result storage

[Copilot]

The MainThreadRunner class used shared instance variables _result and _error accessible by both the main thread and concurrent worker threads. Multiple workers calling execute_on_main_thread simultaneously could overwrite each other's results.

Changes:

• Replaced instance variables with thread-indexed dictionary _results
• Added threading.Lock to protect dictionary access
• Modified run_task slot to accept and store results by thread_id
• Modified execute_on_main_thread to retrieve results using caller's thread ID
• Added explicit existence check before retrieval to avoid masking legitimate None returns

Before:

@pyqtSlot(object, list, dict)
def run_task(self, func, args, kwargs):
    self._result = None  # Shared by all threads
    self._error = None
    try:
        self._result = func(*args, **kwargs)
    except Exception as e:
        self._error = e

After:

@pyqtSlot(object, list, dict, int)
def run_task(self, func, args, kwargs, thread_id):
    result = None
    error = None
    try:
        result = func(*args, **kwargs)
    except Exception as e:
        error = e
    
    with self._lock:
        self._results[thread_id] = (result, error)  # Isolated per thread

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