fix: leaked federation subgraph types on supergraph

[n1ru4l]

Prevent subgraph-specific federation types and scalars being re-declared within the subgraph leaking into the supergraph.

https://linear.app/the-guild/issue/CONSOLE-1643

[gemini-code-assist]

Summary of Changes

Hello @n1ru4l, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request implements a crucial fix for the supergraph composition process, specifically addressing how the FieldSet scalar type is handled. Previously, if FieldSet was explicitly defined within a subgraph's schema, it would erroneously appear in the composed supergraph with an @join__type directive, suggesting it was an external type. The changes ensure that FieldSet is correctly recognized as an internal type in such scenarios, thereby preventing this unintended type leakage and maintaining the accuracy of the supergraph definition.

Highlights

• Bug Fix: Resolves an issue where the FieldSet scalar type, when defined within a subgraph, was incorrectly marked as an external type in the composed supergraph schema.
• New Test Case: Adds a new test to specifically validate that FieldSet does not 'leak' as an external type with a @join__type directive in the supergraph when it is defined internally within a subgraph.

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[gemini-code-assist]

Code Review

This pull request introduces a regression test to ensure that FieldSet defined in a subgraph does not leak into the supergraph. This is a valuable addition to prevent future regressions. The implementation of the test is solid, but I've noticed a minor typo in the test description that I've suggested a fix for.

[github-actions]

🚀 Snapshot Release (alpha)

The latest changes of this PR are available as alpha on npm (based on the declared changesets):

Package	Version	Info
@theguild/federation-composition	0.21.1-alpha-20251210124939-35a8c2d57450dc168b6e786033480b8c5582bf90	npm ↗︎ unpkg ↗︎