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
• 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 26.86567% with 49 lines in your changes missing coverage. Please review.

Project coverage is 67.95%. Comparing base (747c308) to head (880e10b).
Report is 33 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtorc/logic/tablet_discovery.go	0.00%	31 Missing ⚠️
go/vt/topo/faketopo/faketopo.go	50.00%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17718      +/-   ##
==========================================
+ Coverage   67.79%   67.95%   +0.16%     
==========================================
  Files        1587     1586       -1     
  Lines      255829   255209     -620     
==========================================
+ Hits       173427   173433       +6     
+ Misses      82402    81776     -626     

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[GuptaManan100]

Rest looks good to me!

[timvaillancourt]

Update: tested this PR by backporting it to our v19 release

Working as expected ✅. We now get an error log line for the individual cell failure(s), also an error indicating we got a partial result from all cells