[Fusion] Handle non-multipart request when Upload scalar is optional

[tobias-tengler]

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[Copilot]

Pull request overview

This PR fixes handling of optional (Upload) file variables when the incoming request is not multipart (e.g., the variable is null), preventing failures caused by assuming a file lookup is always present.

Changes:

• Make file-upload detection tolerant to missing IFileLookup across Fusion variable coercion and downstream request construction.
• Update Fusion and ASP.NET Core multipart middleware tests to cover nullable/optional upload variables with non-multipart requests.
• Extend test schemas/snapshots to include a nullableUpload field and validate both multipart and non-multipart behaviors.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/HotChocolate/Fusion/test/Fusion.AspNetCore.Tests/FileUploadTests.cs	Adds nullable upload coverage in Fusion tests and a nullable upload resolver.
src/HotChocolate/Fusion/test/Fusion.AspNetCore.Tests/snapshots/FileUploadTests.Upload_Single_File.yaml	Updates schema snapshot to include nullableUpload.
src/HotChocolate/Fusion/test/Fusion.AspNetCore.Tests/snapshots/FileUploadTests.Upload_Single_File_In_Input_Object.yaml	Updates schema snapshot to include nullableUpload.
src/HotChocolate/Fusion/test/Fusion.AspNetCore.Tests/snapshots/FileUploadTests.Upload_Single_File_In_Input_Object_Inline.yaml	Updates schema snapshot to include nullableUpload.
src/HotChocolate/Fusion/test/Fusion.AspNetCore.Tests/snapshots/FileUploadTests.Upload_Nullable_File.yaml	Adds snapshot covering multipart nullable upload.
src/HotChocolate/Fusion/test/Fusion.AspNetCore.Tests/snapshots/FileUploadTests.Upload_Nullable_File_Not_Provided.yaml	Adds snapshot covering non-multipart nullable upload with null variable.
src/HotChocolate/Fusion/test/Fusion.AspNetCore.Tests/snapshots/FileUploadTests.Upload_List_Of_Files.yaml	Updates schema snapshot to include nullableUpload.
src/HotChocolate/Fusion/test/Fusion.AspNetCore.Tests/snapshots/FileUploadTests.Upload_List_Of_Files_In_Input_Object.yaml	Updates schema snapshot to include nullableUpload.
src/HotChocolate/Fusion/test/Fusion.AspNetCore.Tests/snapshots/FileUploadTests.Upload_List_Of_Files_In_Input_Object_Inline.yaml	Updates schema snapshot to include nullableUpload.
src/HotChocolate/Fusion/src/Fusion.Execution/Execution/JsonVariableCoercion.cs	Avoids GetRequired<IFileLookup>() for Upload parsing when no lookup exists.
src/HotChocolate/Fusion/src/Fusion.Execution/Execution/Clients/SourceSchemaHttpClient.cs	Only builds multipart/file-map requests when IFileLookup is present.
src/HotChocolate/Fusion/src/Fusion.Connectors.InMemory/InMemorySourceSchemaClient.cs	Only forwards IFileLookup feature when present.
src/HotChocolate/AspNetCore/test/AspNetCore.Tests/HttpMultipartMiddlewareTests.cs	Adds tests ensuring optional/nullable uploads work when request is not multipart.
src/HotChocolate/AspNetCore/test/AspNetCore.Tests/snapshots/HttpMultipartMiddlewareTests.Upload_Nullable_File.snap	Adds snapshot for nullable upload multipart test.
src/HotChocolate/AspNetCore/test/AspNetCore.Tests.Utilities/UploadQuery.cs	Adds nullable upload resolver and makes optional upload resolver safely return null when not provided.

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