fix(cli): forward nested graphql-config loader options

[ya2s]

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Description

When using GraphQL Config (graphql.config.*), nested extensions.codegen.config values were not forwarded to project.loadSchema / project.loadDocuments.

As a result, options such as inputValueDeprecation: true behaved differently compared to regular codegen.ts config loading.

This change aligns both code paths by forwarding { ...config, ...config.config } for GraphQL Config as well.

Added a regression test to verify nested config values are forwarded.

Related #9659, graphql-hive/graphql-config#1453

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Screenshots/Sandbox (if appropriate/relevant):

N/A

How Has This Been Tested?

• yarn workspace @graphql-codegen/cli test tests/config.spec.ts
• Added regression test: passes nested config values to graphql-config schema loader

Test Environment:

• OS: macOS
• @graphql-codegen/...: @graphql-codegen/cli (workspace)
• NodeJS: v24.11.1

Checklist:

• I have followed the CONTRIBUTING doc and the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Further comments

No additional dependencies are required.

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[ya2s]

@eddeee888 @saihaj @AaronMoat
The root cause of the deprecated field behavior is not fully fixed in this PR.
This change only fixes the GraphQL Config path so that options like inputValueDeprecation are forwarded consistently (same behavior as codegen.ts).