feat(cli): allow disabling legacy federation injection for FederationV2 schemas

[robertobadalamenti]

Description

This PR introduces the disableFederationDirectiveAndScalarInjection flag to the codegen core.

Currently, when federation: true is set, the generator automatically injects legacy Federation v1 definitions (like scalar _FieldSet and directives @key, @Shareable, etc.). In Apollo Federation v2, these are typically imported via the @link directive, causing "Invalid definition" errors during validation.

This flag allows users to opt-out of the automatic injection while keeping federation support active.

Related #10787

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Screenshots/Sandbox (if appropriate/relevant):

https://github.com/robertobadalamenti/codegen-federationv2-bug

How Has This Been Tested?

I have added unit tests in packages/graphql-codegen-core/tests/codegen.spec.ts to verify that:

• The default behavior remains unchanged (injection still happens if the flag is not set).
• When the flag is true, no legacy federation definitions are added to the schema.
• The flag correctly interacts with existing federation settings.

Test Environment:

• OS: macOS
• @graphql-codegen/cli:
• NodeJS: v24.15.0

Further comments

The solution was implemented at the core level to ensure it works across all plugins that rely on the central codegen logic. I considered automatic detection of Federation v2, but an explicit flag was chosen to avoid breaking changes and provide more predictable control to the user.

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[eddeee888]

Thank you @robertobadalamenti !
I feel this is a good quick option for now to support federation v2 👍

Side note: It's a bit odd that this is happening in the core codegen package, so I'll have a think how to decouple it later.