Tablet picker: Handle the case where a primary tablet is not setup for a shard

[rohit-nayak-ps]

Description

During a VReplication workflow, if the workflow has its tablet_types setup as primary, it would panic. This PR fixes that.

This was added a while back, so we should backport to supported versions: https://github.com/vitessio/vitess/pull/14224/files#diff-1bcb2314a21db237d254efff723def4a78db39c34298592d0b0a9a72a58ef575R372

Related Issue(s)

#17571

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

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[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 67.71%. Comparing base (71ccd6d) to head (c0340f3).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/topo/tablet.go	33.33%	2 Missing ⚠️
go/vt/discovery/tablet_picker.go	83.33%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #17573    +/-   ##
========================================
  Coverage   67.71%   67.71%            
========================================
  Files        1584     1585     +1     
  Lines      254721   254833   +112     
========================================
+ Hits       172473   172562    +89     
- Misses      82248    82271    +23     

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[mattlord]

Why use multiple contexts, and why with such short timeouts? Won't this make it flaky in the CI? I get why we use the short context for PickForStreaming() — to cut short the search — but I don't see why we use the other two contexts (instead of one) with such short timeouts.

[rohit-nayak-ps]

It was a cut/paste of a previous test and I had only changed one line, so didn't focus on that. Changed to use a single context.

[mdlayher]

LGTM, thanks!