MINOR: Reduce Test workflow CI time

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This tests the feasible low/medium-risk parts of the Test workflow reduction plan.

What changed:

• avoid duplicate push+PR Test runs on feature branches, while keeping push coverage for main/maintenance branches and tags
• make the Docker Maven cache key depend on build files instead of every Java source edit
• skip the redundant clean goals in ci/scripts/test.sh so tests can reuse the prior build.sh output
• keep the full Docker JDK 17/21/23 matrix on PRs
• add the same WIP guard to integration that the other jobs already use
• gate the expensive integration job by changed paths; non-relevant PRs keep a successful integration check but skip the multi-repo checkout and Docker integration build

Not included: macOS/Windows dependency caching or the build-once/test-many Phase 4. Those should be separate experiments.

Validation:

• PR MINOR: Reduce Test workflow CI time #12 Test workflow passed on run 27478899804 with full JDK 17/21/23 Docker coverage and full integration running because this PR changes the workflow.
• Docs-only control PR MINOR: Docs-only integration gate control #13 validated the skip path: integration completed successfully in 3 seconds and skipped all expensive integration steps.
• Compared with the control PR GH-0: Control CI run #7, non-integration PRs should save roughly the full integration job cost (~25-30 minutes) while preserving a green integration check.