release-25.4: server: wait for full replication in TestCheckRestartSafe_Criticality

[dhartunian]

Backport 1/1 commits from #169990 on behalf of @dhartunian.

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The test asserts zero under-replicated ranges before draining, but
doesn't wait for the cluster to finish initial replication after
startup. Under CI resource pressure (the failing run logged "disk
slowness detected: unable to sync log files within 10s"), ranges may
still be up-replicating when the assertion fires, causing 78
under-replicated ranges where zero were expected.

Add WaitForFullReplication() after cluster start, matching the
pattern already used by TestCheckRestartSafe_RangeStatus and other
tests in the same file. Also remove a duplicate assertion.

Fixes: #167850
Epic: none
Release note: None

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Release justification:

[trunk-io]

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[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes OR fixes for serious issues. Non-production includes test-only changes, build system changes, etc. Serious issues are defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to. Reference the approved ENGREQ ticket in the PR body (e.g., "Fixes ENGREQ-123").

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[cockroach-teamcity]

This change is 

[blathers-crl]

Detected infrastructure failure (matched: self-hosted runner lost communication with the server). Automatically rerunning failed jobs. (run link)

[github-actions]

Reminder: it has been 2 weeks please merge or close your backport!