Extract integration tests into a third parallel CI job

[lubomirw]

Description

Integration tests currently run inside the service / non-service groups. Extract them into their own group and run it as a third parallel CI job alongside service and non-service, using the same mechanism as PR #1491.

PR #1491 changed build_pr.yml (parallel jobs), pom.xml (test grouping), and added CiTestSplitTest.java (architecture guard). Extend those so integration tests form a distinct, mutually-exclusive third group, cutting overall CI wall-clock time.

Definition of Done

• Integration tests removed from service / non-service groups
• Integration tests run as a third parallel CI job alongside service and non-service
• Architecture guard asserts each test lands in exactly one of the three groups
• Overall CI wall-clock time reduced