fix: cast Path to str when spawning tail subprocess

[enlightened88]

Ensure subprocess receives string path when spawning tail to avoid potential cross-platform Path handling issues

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Note

Low Risk
Single-argument type fix when launching a non-critical tail subprocess; minimal behavioral impact beyond improved compatibility.

Overview
Fixes RL log streaming startup by converting the Path for trainer.stdout to a plain string when spawning tail -F via Popen, avoiding Path/subprocess argument handling issues on some platforms.

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