evalengine: Ensure to pass down the precision

[dbussink]

We were failing to pass down the precision correctly in the compiler which would lead to a panic because we'd see an unexpected type downstream.

Marked for backporting as this triggers an internal panic inside vtgate.

Related Issue(s)

Fixes #15610

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

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.07%. Comparing base (e55897b) to head (80d3505).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15611      +/-   ##
==========================================
- Coverage   68.09%   68.07%   -0.02%     
==========================================
  Files        1561     1561              
  Lines      195288   195288              
==========================================
- Hits       132974   132947      -27     
- Misses      62314    62341      +27     

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