fix(browser): strip PYTHONPATH from browser-use child env

[jklance16]

Problem

On Windows, browser_exec (Browser Use mode) crashes with ModuleNotFoundError: No module named pydantic_core._pydantic_core.

Root cause

The browser-use CLI runs in its own Python environment (uv tool / uvx), but Hermes sets PYTHONPATH to its own site-packages in the parent process. A child Python interpreter honors PYTHONPATH ahead of its own site-packages, so browser-use ends up importing pydantic from the Hermes venv — a CPython-3.11 binary that a 3.12 interpreter cannot load (pydantic_core is a compiled extension, platform/version specific).

Fix

Strip PYTHONPATH in _base_subprocess_env() so the CLI resolves its own dependencies regardless of the host Hermes environment.

Verification

• scripts/run_tests.sh on the 3 browser-use test files: 63 passed, 0 failed (10 platform-specific failures excluded — they fail identically before/after this change on Windows because they use POSIX shell-script fake CLIs and POSIX path regexes).
• Manual: env -u PYTHONPATH browser-use --version -> 0.1.8 (fails with the reported error otherwise).

Impact

Windows-only manifestation, but the fix is safe everywhere: the browser-use CLI has never needed the Hermes PYTHONPATH.

[alt-glitch]

This was generated by AI during triage.

Duplicate of #83471. Both remove inherited PYTHONPATH from the browser-use child environment; #83471 is the earlier, broader fix because it also removes PYTHONHOME.

[teknium1]

Thanks @jklance16 — this was a correct, independent fix for the same PYTHONPATH leak, with an excellent root-cause writeup (the 3.11-vs-3.12 pydantic_core ABI detail). #83471 by @n1majne3 arrived a day earlier and also covered PYTHONHOME, so we merged that one via #86371 with first-submitter credit there. Closing this as superseded; the crash class you diagnosed is now fixed on main.