Fix TaskThread crash on missing task record

[ianu82]

Summary

• add a null-check in TaskThread.run() for missing task records
• log the missing record and return early instead of dereferencing None
• add a unit test for the early-return path

Why

If a task is deleted or not found when a thread starts, task_record is None and the previous code raises an AttributeError while setting context fields.

Validation

• python3 -m py_compile mindsdb/interfaces/tasks/task_thread.py tests/unit/interfaces/tasks/test_task_thread.py
• pytest -q tests/unit/interfaces/tasks/test_task_thread.py (fails in this environment due to missing optional dependency mind_castle)

[github-actions]

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[ianu82]

recheck