fix: make DotenvPopulateInput accept NodeJS.ProcessEnv type

[jedwards1211]

fix #767

[Copilot]

Pull Request Overview

This PR fixes a TypeScript type issue where DotenvPopulateInput didn't properly accept the NodeJS.ProcessEnv type. The fix ensures that the processEnv parameter in the config function can accept process.env directly.

• Adds a test case to verify that process.env can be passed to the processEnv parameter
• Validates the type compatibility fix for issue #767

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