Add ECOC26 demo test harness, PMPv1 test topology, and support scripts

Brings over the demo/test-support material this branch's own work has
relied on throughout, none of which touches production service code:

- src/tests/ecoc26_pluggables/: the test harness used to bootstrap the
  T1.1/T1.2/T2.1 pluggable topology, run the ZSM/alarm failure-notification
  tests, and drive the demo end to end (ensure_topology.py, ecoc26_deploy.sh,
  resolve_cluster_env.sh, the ZSM/NBI/pluggable test suites, and the NETCONF
  OC optical mock server used for local testing without real hardware).
- src/tests/PMPv1/: the original point-to-multipoint test topology
  (descriptors, node-agent NETCONF configs, deploy scripts, functional
  create/delete-service tests) that the P2MP work in this branch was
  developed and validated against.
- Root-level scripts: run_ecoc_test.sh / stop_ecoc20_test.sh (start/stop the
  demo test run), free_disk_space.sh (microk8s/docker cleanup utility),
  test_tapi.py (TAPI tooling import sanity check), sync_controller.sh (rsync
  helper for this demo's dev workflow), pytest.ini (registers the
  `integration` marker used by these tests), .gitattributes (Git LFS rules
  for the large binary fixtures under PMPv1/ofc24).
- scripts/monitor_kpis.{sh,py}: watches KPI values flowing through Kafka
  during a demo run.

Excluded: a handful of stray editor/rsync backup files (*.bak_*, *.old)
picked up by the original branch's history, and two clearly-accidental
artifacts (a stale `kubectl get ns -o json` dump and a scratch log file)
that were never meant to be committed.