fix(execpolicy): normalize repeated path separators

[OnlyYu1996]

Summary

• fully collapse repeated path separators in execpolicy path normalization
• normalize Windows separators before sensitive path comparison
• add regression tests for ///etc/shadow, ////etc/passwd, and multi-slash sensitive path policy decisions

Fixes PlatformNetwork/bounty-challenge#53132.

Validation

• cargo +1.90.0 fmt --package cortex-execpolicy --check
• cargo +1.90.0 test -p cortex-execpolicy test_normalize_path_collapses_repeated_slashes
• cargo +1.90.0 test -p cortex-execpolicy test_multi_slash_sensitive_paths_denied
• cargo +1.90.0 test -p cortex-execpolicy
• git diff --check

[greptile-apps]

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