refactor(auth): name completeAuth function

[Fdawgs]

Helps with debugging as this causes it to no longer to be an anonymous function.

See fastify/otel#110

Checklist

• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Copilot]

Pull request overview

This PR refactors the completeAuth function from an anonymous function to a named function expression, improving debugging capabilities by making it identifiable in stack traces and OpenTelemetry instrumentation. This aligns with the issue mentioned in fastify/otel#110 regarding anonymous functions in observability tools.

• Changes the completeAuth function from an anonymous to a named function expression
• Maintains consistency with other function assignments in the Auth constructor (nextAuth, processAuthArray, processAuth)

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