fix: normalize stats model aggregation

[Bortlesboat]

Fixes PlatformNetwork/bounty-challenge#53225.

Summary

• Normalize cortex stats model aggregation keys to lowercase before updating by_model.
• Add a CLI regression test that writes two session files whose model names differ only by case and verifies stats --json reports one merged model row.

Verification

• cargo fmt --check --package cortex-cli
• cargo test -p cortex-cli --test stats_model_case
• cargo check -p cortex-cli
• git diff --check

[greptile-apps]

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