refac: Remove duplicate forAllShards methods from vt/vtctl/workflow

[beingnoble03]

Description

This PR removes some duplicate code from vt/vtctl/workflow

• Removes multiple forAllShards methods
• Moves validateSourceTablesExist from server.go to utils.go removing the method receiver.
• Adds unit tests for validateSourceTablesExist

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]

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[codecov]

Codecov Report

Attention: Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.13%. Comparing base (aba477f) to head (8268a52).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtctl/workflow/resharder.go	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17025      +/-   ##
==========================================
- Coverage   67.15%   67.13%   -0.02%     
==========================================
  Files        1571     1571              
  Lines      251763   251794      +31     
==========================================
- Hits       169080   169053      -27     
- Misses      82683    82741      +58     

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[mattlord]

Just FYI, there are also some unit tests that cover the workflow package in rpc_vreplication_test.go as well. That is needed to test the tablet side (the database queries) of the work where we’ve replaced vexec usage with specific RPCs.

[beingnoble03]

@mattlord thanks for letting me know. I don't think we need unit tests added here then. I think I probably missed that because I just checked coverage for vt/vtctl/workflow and the function was uncovered.