[Fusion] Add support for source schema extensions

[glen-84]

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[Copilot]

Pull request overview

This PR adds support for “source schema extensions” by allowing a *-extensions.graphql* sidecar file to be discovered and appended to a source schema during Fusion composition (CLI and Aspire), with new test resources and coverage verifying the behavior.

Changes:

• CLI: detect and ignore *-extensions.graphql* during directory schema discovery, and append sidecar extensions content when present.
• Aspire: when reading schema from project directory, also append the *-extensions.graphql* sidecar file if it exists.
• Tests/resources: add new SDL/settings/extension fixtures and new test cases validating composition output.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/Nitro/CommandLine/src/CommandLine/Commands/Fusion/FusionCompositionHelpers.cs	Adds *-extensions sidecar detection, directory discovery filtering, and schema+extensions concatenation.
src/HotChocolate/Fusion/src/Fusion.Aspire/SchemaComposition.cs	Adds schema+extensions concatenation when reading schema from project directory.
src/Nitro/CommandLine/test/CommandLine.Tests/Commands/Fusion/FusionComposeCommandTests.cs	Adds CLI tests for composing with extensions (file + directory discovery) and updates resource setup helper.
src/HotChocolate/Fusion/test/Fusion.Composition.Tests/SchemaComposerTests.cs	Adds a composition unit test ensuring extensions are applied during composition.
src/Nitro/CommandLine/test/CommandLine.Tests/resources/valid-extensions/*	Adds new input schema/settings/extensions fixtures for CLI tests.
src/Nitro/CommandLine/test/CommandLine.Tests/resources/valid-extensions-result/composite-schema.graphqls	Adds expected composed composite schema snapshot for extensions scenario.

Comments suppressed due to low confidence (1)

src/Nitro/CommandLine/src/CommandLine/Commands/Fusion/FusionCompositionHelpers.cs:79

• When --source-schema-file points directly to a *-extensions.graphql{s} sidecar, this branch treats it as the primary schema file and then looks for a non-existent *-extensions-settings.json, producing a confusing failure even if the base schema/settings exist. Consider detecting IsExtensionsFile(Path.GetFileName(schemaFilePath)) here and either (a) rejecting with a dedicated, actionable ExitException message, or (b) resolving the corresponding base schema/settings file by stripping the -extensions suffix.

        else if (fileSystem.FileExists(sourceSchemaPath))
        {
            schemaFilePath = sourceSchemaPath;
        }

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