Fix for polymorphic fields in SqlMapping

[milessabin]

[tpolecat]

Can you summarize what this change does? It looks like it's causing all of our discriminator predicates to fail.

[milessabin]

@tpolecat thanks for the update!

[milessabin]

Rebased.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.13861% with 14 lines in your changes missing coverage. Please review.

Project coverage is 75.59%. Comparing base (3a6b7b8) to head (a1dbc5f). Report is 89 commits behind head on main.

Files with missing lines	Patch %	Lines
modules/sql/shared/src/main/scala/SqlMapping.scala	88.00%	6 Missing :warning:
modules/core/src/main/scala/cursor.scala	0.00%	2 Missing :warning:
modules/core/src/main/scala/mapping.scala	89.47%	2 Missing :warning:
modules/circe/src/main/scala/circemapping.scala	80.00%	1 Missing :warning:
modules/core/src/main/scala/queryinterpreter.scala	93.75%	1 Missing :warning:
modules/core/src/main/scala/valuemapping.scala	80.00%	1 Missing :warning:
...les/generic/src/main/scala-2/genericmapping2.scala	75.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #634      +/-   ##
==========================================
+ Coverage   74.22%   75.59%   +1.36%     
==========================================
  Files          32       32              
  Lines        4609     5064     +455     
  Branches     1025     1099      +74     
==========================================
+ Hits         3421     3828     +407     
- Misses       1188     1236      +48     

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