Fix runString temp path to use tmpdir() and add regression test

[dusan-maintains]

Summary

• Fix PythonShell.runString() temp file path generation by calling tmpdir() instead of using the function reference.
• Add a focused unit test that validates temp-file creation in OS temp dir and verifies handoff to PythonShell.run without spawning Python.

Why

• Current code builds filePath with tmpdir + sep + ..., which uses the function object instead of the temp directory path.
• This can produce invalid or non-existent paths for writeFileSync, making runString() fragile.

Test Plan

• npm test -- --grep "temporary file in the OS temp directory"
  • Result in this environment: 1 passing.

Compatibility

• No API changes.
• Existing behavior is preserved; this only fixes temp-path construction and adds regression coverage.