Fix segfault when using Postgres 18.2

[mason-sharp]

Allocate MyProcPort after BackgroundWorkerInitializeConnectionByOid
so that InitPostgres doesn't see a non-NULL MyProcPort and try to
inspect SSL/GSS state on our fake Port (which would segfault).

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch task/pg_18_2

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