EmergencyReparentShard: include SQL thread position in most-advanced candidate selection

[timvaillancourt]

Description

This PR gives the tablet sorter in the EmergencyReparentShard context of how advanced the SQL thread of replicas are. This is used to sort the best candidate when two tablets have the same relay log GTID set

As of this PR, this additional sort dimension won't have any benefit - because the ERS code currently waits for EVERY tablet to apply relay logs, but future PRs I have planned can build on this behaviour, for example the implementation of this RFC: #18529

Related Issue(s)

#18529

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 86.88525% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.52%. Comparing base (6b4fa3d) to head (614da3f).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtctl/reparentutil/replication.go	82.60%	4 Missing ⚠️
go/vt/vtctl/reparentutil/util.go	83.33%	3 Missing ⚠️
go/vt/vtctl/reparentutil/emergency_reparenter.go	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18531      +/-   ##
==========================================
+ Coverage   67.50%   67.52%   +0.02%     
==========================================
  Files        1608     1608              
  Lines      263594   263624      +30     
==========================================
+ Hits       177931   178004      +73     
+ Misses      85663    85620      -43     

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[timvaillancourt]

This PR is ready for another round of reviews now 👍

We've done some internal testing and find this works as planned: which is allow ERS to skip the wait-for-applying-relaylogs phase on a minority of least-advanced tablets

The idea is we know in advance they will never be the most-advanced post-relaylog-apply, so there's no need to wait on them