test: add resolveAppUser unit tests

[adityagautam6102]

Added unit tests for resolveAppUser using vitest, covering user resolution, upsert flow, null githubLogin handling, and database error cases without modifying existing application logic.

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[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

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[Priyanshu-byte-coder]

Merge conflict: vitest.config.ts already exists in main (added by PR #801). Please rebase against main to resolve the conflict.