Backport 26.1: Reject cyclical SDL directive definitions - #4451

Backport of #4385 to the 26.x maintenance branch.

Summary

• reject indirect cycles between SDL directive definitions before schema generation
• detect cycles with an on-demand depth-first traversal instead of precomputing a second reference graph
• use one ordered active path for both cycle detection and diagnostic formatting
• preserve the existing direct self-reference error and detailed indirect cycle message
• consolidate duplicate cycle tests and cover an acyclic directive chain

Fixes #4201

Testing

• ./gradlew clean test — 5,547 tests passed
• focused SchemaTypeDirectivesCheckerTest and SchemaGeneratorTest run — 88 tests passed
• ./gradlew check -x test -x testng