Simplify circular default value validation - #4460

Summary

Ports the core circular-default validation simplifications from the merged 26.x backport in #4450 to master while retaining all existing master integration coverage.

• unify literal and programmatic default traversal in NoDefaultValueCircularRefs
• support programmatic iterable and Java array values through the same traversal
• detect input-object cycles from arguments before external default coercion can recursively expand them
• create fresh stateful schema validation rules for every validation run
• consolidate the lower-level circular-default Spock cases without removing CircularInputDefaultValuesTest

Analysis

The master implementation still had separate AST and external-value algorithms, retained validation-rule state across runs, and could reach external-value coercion before a circular default had been recorded. The #4450 implementation cleanup removes those failure modes and reduces the focused implementation and lower-level test code by 75 net lines.

Unlike #4450, this PR intentionally preserves CircularInputDefaultValuesTest so the higher-level schema-build coverage and explicit SDL argument-default regression remain in place.

Testing

• ./gradlew test --tests graphql.CircularInputDefaultValuesTest --tests graphql.schema.validation.NoDefaultValueCircularRefsTest --tests graphql.schema.validation.SchemaValidatorTest (25 tests)
• ./gradlew test (5,754 tests)
• ./gradlew check -x test -x testng

All passed on Java 25.