VDiff: wait for workflow lock with exponential backoff

[mattlord]

Description

Related Issue(s)

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

AI Disclosure

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

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Backward compatibility

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• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

❌ Patch coverage is 62.06897% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.81%. Comparing base (d411dd0) to head (3ff4c62).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vttablet/tabletmanager/vdiff/table_differ.go	62.06%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18998      +/-   ##
==========================================
- Coverage   69.82%   69.81%   -0.01%     
==========================================
  Files        1610     1610              
  Lines      215303   215322      +19     
==========================================
+ Hits       150328   150331       +3     
- Misses      64975    64991      +16     

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