vtorc: improve handling of partial cell topo results

[timvaillancourt]

Description

This PR improves the safety of getAllTablets in go/vt/vtorc/logic/tablet_discovery.go, which is used to get all tablet records from all cells

The new logic returns the tablets from getAllTablets(...) in a per-cell map (as map[string][]*topo.TabletInfo) to ensure that only the cells that responded are operated on. A list of failed cells is also returned as []string

This avoids tablets from being forgotten when one more cells fail, because before this PR the SQL query to get aliases to forget in refreshTablets(...) does not consider cells that never responded. This bug was introduced by #17388

Related Issue(s)

Closes #17719

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
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• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 25.71429% with 52 lines in your changes missing coverage. Please review.

Project coverage is 67.97%. Comparing base (747c308) to head (4a7128c).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtorc/logic/tablet_discovery.go	0.00%	34 Missing ⚠️
go/vt/topo/faketopo/faketopo.go	50.00%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17718      +/-   ##
==========================================
+ Coverage   67.79%   67.97%   +0.18%     
==========================================
  Files        1587     1586       -1     
  Lines      255829   255212     -617     
==========================================
+ Hits       173427   173470      +43     
+ Misses      82402    81742     -660     

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[GuptaManan100]

Rest looks good to me!

[GuptaManan100]

Is this required?