test: try Github's new parallel build feature

[autonomousapps]

https://github.blog/changelog/2026-06-25-actions-steps-can-now-be-run-in-parallel/
https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax#jobsjob_idstepsparallel

[github-actions]

Job Summary for Gradle

Test :: gradle

Gradle Root Project	Requested Tasks	Gradle Version	Build Outcome	Build Scan®
build-logic	buildHealth	9.6.0	✅
build-logic	check	9.6.0	✅
dependency-analysis-gradle-pl…	buildHealth	9.6.0	❌
testkit	buildHealth	9.6.0	✅
dependency-analysis-gradle-pl…	check	9.6.0	✅
testkit	check	9.6.0	✅
dependency-analysis-gradle-pl…	:functionalTest	9.6.0	❌
dependency-analysis-gradle-pl…	:functionalTest	9.6.0	❌
dependency-analysis-gradle-pl…	:functionalTest	9.6.0	❌
dependency-analysis-gradle-pl…	:checkApi	9.6.0	✅