fix: make stats pricing model match case-insensitive

[hinzga]

Summary

• make built-in stats pricing match against the lowercased model id, matching custom-pricing behavior
• add a regression test confirming GPT-4O and gpt-4o resolve to the same estimated cost

Fixes the upstream behavior described in PlatformNetwork/bounty-challenge#51637.

Testing

• cargo fmt --check --package cortex-cli
• cargo check -p cortex-cli passes; repository emits two existing warnings outside this change
• cargo test -p cortex-cli stats_cmd::tests::test_calculate_cost_is_case_insensitive_for_builtin_pricing is blocked before running this test by an unrelated existing compile error in src/cortex-cli/src/agent_cmd/tests.rs importing private read_file_with_encoding via agent_cmd::loader instead of directly from utils::file

[greptile-apps]

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