fix(stats): support provider-prefixed pricing env keys

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53203.

Summary

• Add a shell-safe double-underscore encoding for provider-prefixed pricing env keys.
• Preserve the existing single-underscore to hyphen mapping.
• Add regression coverage for provider/model IDs and legacy model IDs.

Validation

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

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

[greptile-apps]

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