[Fusion] Properly forward error extensions

[tobias-tengler]

You can see the effect in this diff: db9c72e#diff-b1d15bbb671202d13e06e006faeb5e0824cba82ddec29273bdc26ab8d91c1066

[Copilot]

Pull Request Overview

This PR fixes error extension forwarding in the Fusion GraphQL gateway. The change ensures that error extensions from source schemas are properly passed through to the client by wrapping them in RawJsonValue objects instead of using raw JsonElement values.

• Updates error extension handling to use RawJsonValue wrapper for proper JSON serialization
• Adds a test to verify error extensions are correctly forwarded from source schemas
• Includes snapshot test file for the new test case

Reviewed Changes

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

File	Description
SourceSchemaErrors.cs	Fixed error extension forwarding by wrapping JSON values in RawJsonValue
SourceSchemaErrorTests.cs	Added test case and source schema class to verify error extension forwarding
Error_Extensions_From_Source_Schema_Are_Properly_Forwarded.yaml	Snapshot test file for the new test case

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