Fix vtexplain not handling UNION queries with weight_string results correctly.

[arthurschreiber]

Description

Add special handling for WEIGHT_STRING calls in vtexplain to ensure the result of the function call has the correct type (varbinary instead of int).

Note that the added test cases doesn't actually fail on v18 and newer, but does fail on v17 and older. This is due to changes in evalengine.NullsafeHashcode. But instead of mucking around in the evalengine code trying to backport whatever fixed this, I think it's cleaner to just fix the vtexplain code to return the correct result type for WEIGHT_STRING function calls.

This proposed fix applies cleanly to both affected and unaffected versions, so I feel like adding this change and the unit tests in main first and then backporting to all supported versions is the cleanest approach.

Related Issue(s)

Fixes: #16128

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.29%. Comparing base (8cf7f31) to head (cd3d6f8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16129      +/-   ##
==========================================
+ Coverage   68.22%   68.29%   +0.06%     
==========================================
  Files        1543     1543              
  Lines      197589   197594       +5     
==========================================
+ Hits       134807   134941     +134     
+ Misses      62782    62653     -129     

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[arthurschreiber]

@systay This seems like a regression that has snuck in while the test cases were disabled? 🤔

[arthurschreiber]

I opened #16149 to track this.