Fix TypeError when user not found in api/users.ts

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Summary

Fixed TypeError that occurred when accessing properties on undefined users in the Users API.

Changes

• Updated getUserById return type to User | undefined to properly indicate it may not find a user
• Added optional chaining (?.) in getUserEmail and isAdmin to handle undefined users gracefully
• Updated return types for getUserEmail to string | undefined
• Fixed isAdmin to return false for non-existent users instead of throwing
• Cleaned up unused imports (fs) and console.log statements
• Fixed code formatting and indentation issues in formatUserName
• Updated tests to properly assert undefined behavior instead of expecting no throws

Tests

All tests now pass:

• ✅ getUserById handles non-existent user IDs
• ✅ getUserEmail returns undefined for non-existent users
• ✅ isAdmin returns false for non-existent users
• ✅ All existing tests continue to pass

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This agent session was co-authored by bekah-hawrot-weigel and Continue.