feat: part 2 Env new behavior: OS Fallback and alert

[royendo]

follow up to #8636

https://www.loom.com/share/5041c12bf37740f190eac80b66fbc4e8

want to add fallback to OS vars and add warning in UI for such

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[royendo]

Code Review

Professionalism & Code Quality: ⭐⭐⭐⭐ (Good)

Strengths:

• Good addition of OsEnvVariables field to track which variables came from OS environment
• Clean proto definition with proper documentation

Issues to Address:

• The proto changes suggest there's UI work to display warnings, but I only see the proto changes in this diff. Ensure the frontend warning display is included or tracked separately.

E2E Testing: ⚠️ Missing

No E2E tests visible in this diff.

Recommendations:

• Add E2E test verifying the warning is displayed when OS env variables are used
• Test that the warning helps users understand they need to add variables to .env for deployment

General Comments:

• This pairs well with PR feat: .env UI editor #8520 (env UI editor)
• Good safety feature to help users avoid deployment surprises