Backport of #4441 to the 26.x maintenance branch.

The original commit applies without changes.

Summary:

• makes fragment cycle validation linear
• avoids repeatedly copying fragment paths while retaining validation behavior
• adds focused coverage for large fragment graphs

Testing:

• ./gradlew test --tests graphql.validation.NoFragmentCyclesTest (15 tests)
• ./gradlew check -x test -x testng javadoc