luigi: silence 'not of type string' warnings on run_id_dir / callback

[neoneye]

Summary

Production logs emitted two UserWarnings on every Luigi task instantiation:

Parameter "run_id_dir" with value "/app/run/…" is not of type string.
Parameter "_pipeline_executor_callback" with value "<bound method …>" is not of type string.

Both were declared as plain luigi.Parameter while holding a pathlib.Path and a bound method.

Fix

New worker_plan_internal/luigi_util/parameters.py:

• PathParameter — accepts str | Path, normalizes/parses to Path, serializes to str.
• CallableParameter — non-significant private slot for bound methods.

Both subclass luigi.Parameter, which on its own silences the string-type warning: Luigi's Parameter._warn_on_wrong_param_type returns early when self.__class__ != Parameter.

Wire-up:

• PlanTask.run_id_dir → PathParameter
• PlanTask._pipeline_executor_callback → CallableParameter
• InitialPlanRawTask.run_id_dir → PathParameter (the warning also fired here once worker_plan: represent plan_raw.json as InitialPlanRawTask #666 introduced the ExternalTask, since self.clone(InitialPlanRawTask) propagates the parent's Path).

Test plan

• Generate a plan and confirm the two UserWarning lines no longer appear in the logs.
• Spot-check that self.run_id_dir / FilenameEnum.X.value still works in tasks that read it as a Path.

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