Move VDiff related workflow server APIs to vdiff.go and add unit tests

[beingnoble03]

Description

This PR

• moves VDiff related workflow Server APIs from server.go to vdiff.go
• includes moving the related unit tests to vdiff_test.go
• adds previously missing unit tests for VDiffDelete

The change in code coverage:
Code coverage on main:

go test ./go/vt/vtctl/workflow --coverprofile=c.out
ok  	vitess.io/vitess/go/vt/vtctl/workflow	2.753s	coverage: 66.4% of statements

Code coverage on vdiff-refactor:

go test ./go/vt/vtctl/workflow --coverprofile=c.out
ok  	vitess.io/vitess/go/vt/vtctl/workflow	2.772s	coverage: 66.7% of statements

Related Issue(s)

• Tracking Issue: vtctldclient/vtctld: Enhance test coverage and code refactor  #16499

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

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

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

Attention: Patch coverage is 85.00000% with 36 lines in your changes missing coverage. Please review.

Project coverage is 67.67%. Comparing base (9383943) to head (80454e7).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtctl/workflow/vdiff.go	85.00%	36 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17466   +/-   ##
=======================================
  Coverage   67.67%   67.67%           
=======================================
  Files        1583     1584    +1     
  Lines      254363   254368    +5     
=======================================
+ Hits       172140   172149    +9     
+ Misses      82223    82219    -4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[mattlord]

We want to remove all of this commented code that was removed, don't we?