fix(sessions): support cross-platform path separators in is_anonymous_session

[zariffromlatif]

Summary

Fixes an issue where \is_anonymous_session()\ checked only \os.path.sep\ (\\\ on Windows), causing relative session paths using forward slash separators (e.g. \path/to/session.json) to be incorrectly classified as named host sessions on Windows.

Changes

• Updated \is_anonymous_session()\ to check both \os.path.sep\ and \os.path.altsep\ (\os.path.sep in session_name or bool(os.path.altsep and os.path.altsep in session_name)).
• Added unit test \ est_is_anonymous_session_cross_platform\ verifying POSIX, Windows, and named session inputs.

Verification

• 62/62 session tests passing locally (\pytest tests/test_sessions.py).

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