🧪 Add tests for getModelPrice

[the-abra]

🎯 What: The getModelPrice function in src/model-cost.ts was previously untested. This function handles dictionary lookup for provider/model pricing, and is critical for cost calculations.
📊 Coverage: Added test suite in tests/model-cost.test.ts to cover exact matches, prefix/partial matches (both model starting with key and key starting with model), and fallback behavior (provider default and absolute default).
✨ Result: Improved test coverage and reliability for cost calculation utilities.

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PR created automatically by Jules for task 7511890374774044020 started by @the-abra

[google-labs-jules]

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