Fix dual merging in outer join queries

[GuptaManan100]

Description

This PR resolves the issue described in #15958.

The problem was that we were merging the dual query with the sharded route, and each of the shards ended up returning one row. We should not be merging dual queries into sharded routes. We can only merge them if the other side is a single shard routing.

Related Issue(s)

• Fixes Bug Report: Dual left join query returns more results than it should #15958

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
• 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

All modified and coverable lines are covered by tests ✅

Project coverage is 68.24%. Comparing base (f118ba2) to head (84c968f).
Report is 167 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15959      +/-   ##
==========================================
- Coverage   68.40%   68.24%   -0.17%     
==========================================
  Files        1556     1543      -13     
  Lines      195121   197592    +2471     
==========================================
+ Hits       133479   134845    +1366     
- Misses      61642    62747    +1105     

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[harshit-gangal]

Is this not a problem only with left join and that too when the dual table is on the left hand side of the join.

[harshit-gangal]

Is this not a problem only with left join and that too when the dual table is on the left hand side of the join.

I see the code follows this rule, it was missing in the PR description.

[GuptaManan100]

@harshit-gangal @systay Had to change quite a few things to get everything to work. Could I request you two to review it again?

A lot of code has changed since the approval.

[GrahamCampbell]

I think the milestone on this issue is possibly incorrect. This was merged to v21, but the milestone says v20.