CODEGEN-479 - Fix client preset not working with exactOptionalPropertyTypes=true when documentMode=string

[eddeee888]

Description

When exactOptionalPropertyTypes=true is set in tsconfig, we'd get the following error:

Class 'TypedDocumentString<TResult, TVariables>' incorrectly implements interface 'DocumentTypeDecoration<TResult, TVariables>'.
  Types of property '__apiType' are incompatible.
    Type '((variables: TVariables) => TResult) | undefined' is not assignable to type '(variables: TVariables) => TResult'.
      Type 'undefined' is not assignable to type '(variables: TVariables) => TResult'.ts(2420)

This is because DocumentTypeDecoration<TResult, TVariables>['__apiType'] type resolves to ((variables: TVariables) => TResult) | undefined but only (variables: TVariables) => TResult is valid here.

By wrapping it with NonNullable, we are removing the | undefined from DocumentTypeDecoration<TResult, TVariables>['__apiType'], meaning the implementing type now matches up.

Related #10021

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Unit test

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🦋 Changeset detected

Latest commit: 9b12d11

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@graphql-codegen/typed-document-node	Patch
@graphql-codegen/client-preset	Patch

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🚀 Snapshot Release (alpha)

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

Package	Version	Info
@graphql-codegen/typed-document-node	5.1.2-alpha-20250623123201-9b12d112d561982456dc33390d36ce8420987001	npm ↗︎ unpkg ↗︎
@graphql-codegen/client-preset	4.8.3-alpha-20250623123201-9b12d112d561982456dc33390d36ce8420987001	npm ↗︎ unpkg ↗︎