Re-port of PR #35464 onto the rewritten hermetic conftest:
- autouse _neutralize_webbrowser fixture records open/open_new/open_new_tab
and webbrowser.get() instead of launching a real browser
- autouse _neutralize_macos_keychain_creds defaults the Anthropic Keychain
reader to None, with an opt-in allow_macos_keychain marker
- regression tests in tests/test_hermetic_side_effect_guards.py
- tests/agent/test_anthropic_keychain.py opts in via pytestmark
Salvaged-from: #35464
Co-authored-by: y0shualee <yuxiangl490@gmail.com>
