feat(evals): add session-level pii_detection evaluator (#15096)

Co-authored-by: Elizabeth Hutton <42793432+ehutt@users.noreply.github.com>
Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>