Add tests for environment helpers

[abrarshivani]

Description

Add focused unit coverage for path-list prepending and environment variable replacement. The cases verify ordering, no-op behavior, and exact key matching so similarly prefixed variables remain untouched.

Checklist

• No secrets, sensitive information, or unrelated changes
• Lint checks passing (make lint)
• Generated assets in-sync (make validate-generated-assets)
• Go mod artifacts in-sync (make validate-modules)
• Test cases are added for new code paths

Testing

• make fmt
• Unit tests pending

Link to Devin session: https://nvidia-cloud.devinenterprise.com/sessions/202583d9612e47d987e4d776b7aa9225
Open in Devin Desktop: https://nvidia-cloud.devinenterprise.com/desktop/session/202583d9612e47d987e4d776b7aa9225?variant=devin
Requested by: @abrarshivani

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring

[copy-pr-bot]

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