fix: reference table join merge

[harshit-gangal]

Description

This PR fixes the join merging logic for two reference tables. The merge should also handle the alternative routes of the reference table during merging.

This needs to be backported to all the supported versions as it causes planner time panic.

Related Issue(s)

• Fixes Bug Report: Joining too many reference tables causes panic #15799

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]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 68.63%. Comparing base (59408f9) to head (8db33da).
Report is 4 commits behind head on main.

Files	Patch %	Lines
go/vt/vtgate/planbuilder/operators/join_merging.go	93.33%	1 Missing ⚠️
go/vt/vtgate/planbuilder/operators/joins.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16488      +/-   ##
==========================================
- Coverage   68.64%   68.63%   -0.01%     
==========================================
  Files        1551     1551              
  Lines      199471   199530      +59     
==========================================
+ Hits       136925   136957      +32     
- Misses      62546    62573      +27     

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