CodeQL doesn't model Path.iterdir() as a taint-clearing enumeration the
way it does os.scandir() -- _trusted_plugin_dir's iterdir-based rebuild
still traced plugin_id through to the manifest.json open(). Switched to
scandir, matching the pattern already verified clean on PR #396.
Also stops surfacing raw exception text (update()/display() failures)
in the JSON render response -- logs full detail server-side via
exc_info instead, returning only the exception class name to the
client. And drops path values from three plugin_loader debug/error
logs that CodeQL flags as clear-text-logging of externally-influenced
data, keeping plugin_id (not flagged) for context.
