Fix System.Net.Http.Json tests in AOT

[eiriktsarpalis]

I have verified locally that enabling reflection makes the test pass in AOT.

Fix #89195.

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Issue Details

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Fix #89195.

Author:	eiriktsarpalis
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-

[eerhardt]

LGTM. Have we thought about how we are testing in AOT with JsonSerializerIsReflectionEnabledByDefault=false (which is the default for AOT)?

[eiriktsarpalis]

Have we thought about how we are testing in AOT with JsonSerializerIsReflectionEnabledByDefault=false (which is the default for AOT)?

At some point we'd need to refactor the test suite so that reflection vs. non-reflection components are adequately segregated. The tests happen to work in AOT currently because this declaration ensures the right generic specializations are included with the app, so re-enabling reflection seemed like the simplest way to make tests green again.