[WIP] Upgrade to Spring Boot 4.2, graphql-java 26.+, spring-graphql 2.1

[jiholee17]

[Work in progress, do not merge]

Summary

Updates DGS Framework for the Spring Boot 4.2 line, GraphQL Java 26.+, and Spring GraphQL 2.1.

Spring Boot 4.2: https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-4.2-Release-Notes
GraphQL Java 26: https://github.com/graphql-java/graphql-java/releases/tag/v26.0
Spring GraphQL 2.1: https://github.com/spring-projects/spring-graphql/wiki/Spring-for-GraphQL-2.1

Changes

• Updated Spring Boot from 4.0.0 to 4.2.0-M2.
• Updated GraphQL Java from 25.0 to 26.1.
• Updated Apollo Federation JVM support from 5.3.0 to 7.0.0.
• Updated GraphQL Java Extended Validation from 22.0 to 24.0.
• Refreshed dependency locks across affected modules.
• Updated Kotlin production and test code for GraphQL Java 26’s JSpecify nullability annotations.
• Updated instrumentation implementations for nullable InstrumentationState and execution metadata.
• Updated federation entity error-path expectations.
• Added safe fallback metric tags for incomplete execution metadata.
• Updated tests for nullable GraphQL results, context values, schema lookups, and Spring beans.

Compatibility status

Completed

• Updated DGS production code for GraphQL Java 26 nullability contracts.
• Updated affected Kotlin tests and instrumentation implementations.
• Updated federation entity error-path expectations to paths such as ["/_entities", 0].
• Verified existing federation, validation, metrics, pagination, and Spring GraphQL tests.
• Updated Federation JVM to a version compatible with GraphQL Java 26.
• Refreshed affected dependency locks.

Not yet explicitly addressed

• Decide whether DGS should retain, configure, document, or disable GraphQL Java 26’s default query limits:
  • Maximum depth: 100
  • Maximum field count: 100,000
• Add tests for MaxQueryDepthExceeded and MaxQueryFieldsExceeded.
• Audit schemas for the stricter GraphQL Java 26 validation rules.
• Add regression coverage for built-in directive presence and ordering.
• Add explicit coverage for Federation v2.13–v2.15 features.
• Document downstream Kotlin migration requirements for custom instrumentation and direct GraphQL Java API usage.
• Review and document the federation error-path change as a client-visible compatibility consideration.
• Repeat the compatibility review as new Spring Boot milestones, release candidates, and final releases become available.