Fix #6545 sorting variables for expression object sort fields

[michaelstaib]

• add a regression test for sorting via variables on a custom expression sort field returning an object type without a default constructor
• avoid assigning the expression return CLR type as the input field runtime type in sorting descriptors
• preserve sorting expression runtime metadata by deriving SortField.RuntimeType from the expression return type

Fixes #6545
Closes #6660

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (84e5c32) to head (a5ba417).
⚠️ Report is 54 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff      @@
##   main   #9250   +/-   ##
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[Copilot]

Pull request overview

This PR fixes a variable-coercion failure when sorting via variables on a custom expression-based sort field that returns an object type without a parameterless constructor (issue #6545). It ensures the input parsing runtime type is not incorrectly set to the expression’s CLR return type while still preserving the expression runtime metadata needed by the sorting pipeline.

Changes:

• Added a regression test reproducing sorting-by-variable on an expression sort field returning a non-default-constructible object.
• Stopped assigning SortFieldConfiguration.RuntimeType from an expression’s CLR return type to avoid incorrect variable parsing behavior.
• Preserved sorting runtime metadata by deriving SortField.RuntimeType from the expression return type during type completion.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/HotChocolate/Data/test/Data.Sorting.InMemory.Tests/Issue6545ReproTests.cs	Adds regression coverage for sorting with variables on an expression sort field returning an object without a default ctor.
src/HotChocolate/Data/src/Data/Sorting/SortFieldDescriptor.cs	Prevents expression return CLR type from being used as the input-field runtime type in the descriptor configuration.
src/HotChocolate/Data/src/Data/Sorting/Fields/SortField.cs	Restores expression return-type runtime metadata at the SortField level for sorting execution while keeping input parsing safe.

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