[e2e] Add management-side agent-network scenarios (Pillar 2)

Port the API-driven agent-network scenarios from the bash suites to Go,
sharing one combined server per package run (TestMain) with each test
owning its resource cleanup. Drives the /api/agent-network/* endpoints
through the shared REST client's NewRequest primitive with the generated
api types.

Scenarios:
- provider lifecycle (create/get/list/delete + 404 after delete)
- provider validation (missing api_key, unknown catalog id → 4xx)
- settings collection-toggle round-trip with cluster/subdomain immutability
- policy window floor (reject <60s enabled limit, accept at 60s)
- consumption read endpoint returns an array

All deterministic and dependency-free (dummy provider keys; no upstream
calls), so they run headless in CI.

e2e/agentnetwork/main_test.go

@@ -0,0 +1,46 @@
+//go:build e2e
+
+// Package agentnetwork holds the container-based agent-network e2e suite. A
+// single combined server is built and bootstrapped once per package run
+// (TestMain) and shared across tests via srv; each test creates and cleans up
+// its own resources so order doesn't matter.
+package agentnetwork
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/netbirdio/netbird/e2e/harness"
+)
+
+// srv is the shared combined server for the package, ready (PAT-authenticated)
+// by the time any Test runs.
+var srv *harness.Combined
+
+func TestMain(m *testing.M) {
+	os.Exit(run(m))
+}
+
+func run(m *testing.M) int {
+	// Generous timeout to cover a cold image build on first run.
+	ctx, cancel := context.WithTimeout(context.Background(), 15*time.Minute)
+	defer cancel()
+
+	var err error
+	srv, err = harness.StartCombined(ctx)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "e2e: start combined server: %v\n", err)
+		return 1
+	}
+	defer func() { _ = srv.Terminate(context.Background()) }()
+
+	if _, err := srv.Bootstrap(ctx); err != nil {
+		fmt.Fprintf(os.Stderr, "e2e: bootstrap admin PAT: %v\n", err)
+		return 1
+	}
+
+	return m.Run()
+}