fix(stats): ignore usage-only session files

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53202.

Summary

• Ignore top-level session usage aggregates when the session has no messages array entries.
• Add a regression test for usage-only session JSON so stats no longer report non-zero tokens/cost with otal_messages: 0.

Validation

• cargo +1.90.0 fmt --package cortex-cli --check
• git diff --check

Targeted cargo +1.90.0 test -p cortex-cli test_parse_session_file_ignores_usage_without_messages -- --nocapture is blocked in this Windows/MSVC environment before crate tests run by the existing �ws-lc-sys C build failure.

[greptile-apps]

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