make sure to handle unsupported collations well

[systay]

Description

We were not handling unsupported collations well in semantic analysis and during planning.

Related Issue(s)

Fixes #15133

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

[vitess-bot]

Review Checklist

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General

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Tests

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Documentation

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Backward compatibility

• Protobuf changes should be wire-compatible.
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• 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.

[harshit-gangal]

CI failing on Unit Tests

[codecov]

Codecov Report

Attention: 135 lines in your changes are missing coverage. Please review.

Comparison is base (eddb39e) 47.29% compared to head (341d872) 70.61%.
Report is 106 commits behind head on main.

Files	Patch %	Lines
go/vt/mysqlctl/builtinbackupengine.go	12.90%	54 Missing ⚠️
go/vt/mysqlctl/backupengine.go	0.00%	20 Missing ⚠️
go/vt/mysqlctl/xtrabackupengine.go	0.00%	15 Missing ⚠️
go/vt/schemadiff/column.go	70.83%	7 Missing ⚠️
go/mysql/conn.go	53.84%	6 Missing ⚠️
go/vt/mysqlctl/backup.go	37.50%	5 Missing ⚠️
go/mysql/capabilities/capability.go	91.48%	4 Missing ⚠️
go/mysql/query.go	77.77%	4 Missing ⚠️
go/vt/mysqlctl/schema.go	72.72%	3 Missing ⚠️
go/vt/schemadiff/schema.go	92.10%	3 Missing ⚠️
... and 10 more

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #15134       +/-   ##
===========================================
+ Coverage   47.29%   70.61%   +23.31%     
===========================================
  Files        1137     1376      +239     
  Lines      238684   182418    -56266     
===========================================
+ Hits       112895   128811    +15916     
+ Misses     117168    53607    -63561     
+ Partials     8621        0     -8621     

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[GrahamCampbell]

This is in the v19.0.0 milestone, but was merged to 20.x. Will this be backported?

[systay]

This is in the v19.0.0 milestone, but was merged to 20.x. Will this be backported?

You keep me on my toes, @GrahamCampbell 😁

Here it is: #15143