chore: Remove redundant processing from exprToProtoInternal

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

We have two very similar methods, exprToProto and exprToProtoInternal.

exprToProto performs a transformation on the expression tree to handle decimal promotion and then calls into the recursive method exprToProtoInternal to perform the translation to protobuf.

Within exprToProtoInternal we should only recurse into exprToProtoInternal but in some cases we were calling exprToProto instead,. This was not harmful but is performing redundant work.

What changes are included in this PR?

• Improve documentation
• Replace calls to exprToProto with exprToProtoInternal within exprToProtoInternal

How are these changes tested?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 39.09%. Comparing base (f09f8af) to head (a51967a).
Report is 11 commits behind head on main.

Additional details and impacted files

@@              Coverage Diff              @@
##               main    #1351       +/-   ##
=============================================
- Coverage     56.12%   39.09%   -17.04%     
- Complexity      976     2065     +1089     
=============================================
  Files           119      260      +141     
  Lines         11743    60269    +48526     
  Branches       2251    12834    +10583     
=============================================
+ Hits           6591    23562    +16971     
- Misses         4012    32226    +28214     
- Partials       1140     4481     +3341     

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[parthchandra]

lgtm