test : add unit tests for resolveAppUser function

[tmdeveloper007]

Summary of What Needs to be Done:
Add unit tests for the resolveAppUser function in src/lib/resolve-user.ts. Tests must use vitest framework and import actual source code.

Changes that Need to be Made:

• Create test using vitest describe/it/expect
• Import resolveAppUser from src/lib/resolve-user.ts
• Test returns existing user when found
• Test creates new user when not found
• Test returns null on failure

Impact that it would Provide:

• Ensures user resolution logic is correct
• Validates upsert behavior

[Harsh-Codes-77]

/assign

[github-actions]

✅ Closed as completed!

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