Add NetBird server to test connections (#328)

This change adds a NetBird server to the e2e test and ensures that
cluster proxy peers can start and connect to the server.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Changes**
* Updated the `ClusterProxy` `spec.apiServer` default to include a
trailing `/` when omitted, aligning operator behavior with the
Kubernetes in-cluster API URL.
* **Documentation**
* Refreshed the `ClusterProxySpec.apiServer` API reference to reflect
the trailing `/` default.
* **Tests**
* Improved end-to-end coverage with a dynamically provisioned management
URL, more robust readiness polling, and enhanced `ClusterProxy`
validation.
* **Chores**
* Updated the end-to-end test Go configuration and CI to use the
e2e-specific Go settings/toolchain.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

.github/workflows/go.yaml

@@ -48,6 +48,12 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@4a3601121dd01d1626a1e23e37211e3254c1c06c #v6.4.0
         with:
-          go-version-file: go.mod
+          go-version-file: test/e2e/go.mod
+      - run: docker inspect $(docker network ls -q)
       - name: Run e2e tests
         run: make -B test-e2e
+      - name: Debug on failure
+        if: failure()
+        run: |
+          docker ps -a
+          docker inspect $(docker ps -a -q)