Fix "no replicas available" sql hint to be conditional based on managed or unmanaged cluster

[chaas]

Fix "no cluster replicas available" adapter sql hint to be conditional based on managed or unmanaged cluster, as reported in https://github.com/MaterializeInc/materialize/issues/20000#issuecomment-2197198856.

Motivation

• This PR fixes a recognized bug.

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: