Consolidate duplicate projection and serialization code

[iuliiasobolevska]

Summary

• consolidate the shared projection class scaffold and root constructor generation
• remove the unused ClientApiGenerator document property
• keep input-object serialization in InputValueSerializer while reducing NullableInputValueSerializer to its null-inclusion policy

Verification

• ./gradlew :graphql-dgs-codegen-shared-core:test --no-scan
• ./gradlew :graphql-dgs-codegen-core:test --no-scan