🧪 Add tests for generateTypeDefinitions

[KevinArce]

This PR adds unit tests for the generateTypeDefinitions function in src/writer.ts.

🎯 What:

The testing gap addressed was the lack of unit tests for the generateTypeDefinitions function, which is responsible for creating TypeScript declaration files for environment variables.

📊 Coverage:

The following scenarios are now tested:

• Valid Content Generation: Verifies that the function generates the correct TypeScript boilerplate and includes the provided keys as string properties.
• Deterministic Output: Verifies that keys are sorted alphabetically in the output file, ensuring consistent results regardless of input order.
• Directory Creation: Verifies that the function correctly creates missing parent directories for the specified output path.
• Empty Keys Handling: Verifies that the function handles empty input correctly by generating a valid but empty ProcessEnv interface.

✨ Result:

Increased test coverage for the writer module and improved reliability of the type definition generation logic. The tests were verified to pass using bun test and their ability to catch bugs was confirmed by temporarily breaking the sorting logic.

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PR created automatically by Jules for task 15834586361286711632 started by @KevinArce

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