ci: parallelize e2e tests with BUILD_ID-scoped resources

[Kukkerem]

Closes #274

Each Test* function now runs as its own matrix entry in GitHub Actions, so the e2e suite finishes at the time of the slowest test instead of the sum of all tests. Every test-created CF resource name gets a -$BUILD_ID suffix (except the role-import usernames - see Trade-offs) so parallel jobs and concurrent workflow runs don't collide on the shared cf-ci-e2e org.

What changed

• GitHub Actions discovers Test* functions via grep and fans them out (fail-fast: false)
• CF resource names are BUILD_ID-scoped via runScopedName() / NewID() - CRS YAMLs are rendered into .work/rendered-crs/ with envsubst '$BUILD_ID', tracked files stay clean
• BUILD_ID is a workflow-level env in CI (run_id-run_attempt) - make test-acceptance defaults to HHMMSS - cleanup.go falls back to 0000 when invoked manually without it
• New cleanup.go deletes BUILD_ID-scoped resources after the matrix finishes - checks out github.base_ref (not the PR head) so PR authors can't inject code that runs with CF credentials
• concurrency: e2e-cf-ci-e2e (cancel-in-progress: false) queues concurrent workflow runs - orgrole/spacerole import tests use static sap.ids usernames that can't be BUILD_ID-scoped without IDS provisioning
• Removed resetTestOrg and the sequential-era CF-cleanup helpers from test_env.go (they'd race in parallel - cleanup.go does the job now)
• Pre-provisioned CF fixtures (cf-ci-e2e, upgrade-test-org, *-donotdelete spaces, upgrade-test-import-space, system domains, sap.ids test users) are Observe-only and untouched

Trade-offs

• PR fixes to cleanup.go only take effect after merge - base-ref checkout is a deliberate security choice (matches the BTP provider)
• Concurrent workflow runs serialize instead of parallelizing - alternative would need a user pool or shadow-user creation in SAP IDS
• Cleanup is best-effort - canceled runs leave resources until the next pick-up

The valid e2e test run:
https://github.com/SAP/crossplane-provider-cloudfoundry/actions/runs/26230189808

[SatabdiG]

I really like this, the security stuff especially 🔥 Just some minor comments

[SatabdiG]

lgtm! thanks for taking this up, much needed 🦸

[SatabdiG]

lgtm