fix: return 404 for missing users

[krowvin]

Summary

• return a CDA 404 Not Found error when GET /users/{user-name} cannot find the requested user
• document the 404 response in OpenAPI
• add an authenticated integration test for the missing-user case
• include the required Swagger UI screenshot

Root cause

UserDao.getByUniqueName(...) returns an empty Optional when the user does not exist. The controller converted that to null and passed it to Formats.format, whose non-null check raised the NullPointerException that CDA reported as a 500. The controller now converts the empty result to NotFoundException, allowing CDA's existing error handling to return a structured 404 response.

Verification

• compileJava and compileTestJava
• checkstyleMain and checkstyleTest
• focused UserManagementTestIT.test_get_missing_user_not_found integration test
• isolated Docker Compose end-to-end check:
  • existing user returned 200
  • missing user returned 404 with User not found: DOES_NOT_EXIST

• AI tools used

[krowvin]

I found this error when fishing for a stack trace for
HydrologicEngineeringCenter/cwms-cli#247