storage: Fix for runtime invalid-socket-address error

[rjobanp]

Motivation

Postgres connections over AWS PrivateLink were broken by #26186 which introduced a runtime error because tokio::net::lookup_host here requires a port provided:

materialize/src/postgres-util/src/tunnel.rs

Lines 305 to 306 in 80e3fe9

	let privatelink_host = mz_cloud_resources::vpc_endpoint_name(*connection_id);
	let privatelink_addrs = tokio::net::lookup_host(privatelink_host).await?;

We unfortunately did not have a PrivateLink + Postgres Source test in CI. This work is tracked here https://github.com/MaterializeInc/cloud/issues/8238 and will be prioritized for implementation during the next 2 weeks (cc @jubrad @pH14 @benesch )

I reproduced this error on my staging environment using a debug build (from #26804) and then verified this fix with a second debug build (from #26805)

This also fixes the thiserror conversion for PostgresError that caused us to originally attribute the error to something related to ssh-connections.

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:

[guswynn]

I feel like tokio's ToSocketAddr's has a broken api that it lets this happen!