CSHARP-5918: ExpressionNotSupportedException when a $set stage expression uses ToList method

[sanych-sun]

The problem was caused by the ToListMethodTranslater which used to ignore the serializerMap produced by the serializer finder and therefore used the wrong serializer.

[Copilot]

Pull request overview

Fixes ExpressionNotSupportedException when a $set stage expression uses ToList() by replacing the manually constructed ListSerializer with the serializer obtained from the translation context (which respects the serializer map).

Changes:

• Fixed ToListMethodToAggregationExpressionTranslator to use context.GetSerializer(expression) instead of manually creating a ListSerializer, ensuring the correct serializer is used
• Added integration tests covering ToList() in $set pipeline updates, both same-model and cross-model scenarios

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
ToListMethodToAggregationExpressionTranslator.cs	Use context-provided serializer instead of manually constructing one
CSharp5918.cs	New integration tests for the fix

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