Fix panic in user defined aggregation functions planning

[GuptaManan100]

Description

This PR fixes the panic found in #16397.
On investigation it was found that this panic was introduced in the changes in #16049. This PR fixes the panic and also adds test cases to verify we are able to plan udf queries properly and fail in case we are unable to push them down to MySQL.

Related Issue(s)

• Fixes Bug Report: Panic in query planning of a query using user defined aggregation function #16397

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

All modified and coverable lines are covered by tests ✅

Project coverage is 68.67%. Comparing base (fd6411d) to head (f9e7b88).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16398      +/-   ##
==========================================
- Coverage   68.68%   68.67%   -0.01%     
==========================================
  Files        1548     1548              
  Lines      199063   199087      +24     
==========================================
+ Hits       136721   136727       +6     
- Misses      62342    62360      +18     

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[systay]

Nice fix!