ReverseTraffic bug fix: allow reversing reads when only reads have been switched

[rohit-nayak-ps]

Description

There is a regression from vtctl to vtctld where, if only read traffic has been switched for a VReplication Workflow, attempting to reverse this switch errors out. If write traffic has also been switched it works fine. Reported here: #16918.

The root cause is that, for a workflow which has been (partially) switched the code deduces the state from the reverse workflow, which is not created unless writes are switched.

Due to paucity of time, aiming to get this into the v21 release we start with a hackier interim fix in this PR.

This will be followed by a refactored implementation from: #16920 which has some significant backports and can bake in v22.

This has been present from day 0, so we backport to all supported versions: patches for the older releases are expected in the next couple weeks.

Related Issue(s)

#16918

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]

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[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.03%. Comparing base (e881b9f) to head (c32c7e0).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtctl/workflow/server.go	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16998      +/-   ##
==========================================
- Coverage   67.04%   67.03%   -0.01%     
==========================================
  Files        1571     1571              
  Lines      251677   251683       +6     
==========================================
- Hits       168729   168723       -6     
- Misses      82948    82960      +12     

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[rohit-nayak-ps]

The alternate fix turned out to touch a few code paths, so the aim is to try to get #16920 into RC2.