proxmox: contract and observability tests, integration test, testing docs

The contract sweep in provider_test.go: every method and Check returns
within a second of a cancel, every wrapErr path carries the configured
name, op and id, the V0/V1/V2 logging tiers and the no-op lines, and a
Get over the real client whose HTTP spans are children of provider.get.
proxmox_integration_test.go (-tags=integration) holds TestProxmox_lifecycle
and TestProxmoxCheck_realHost; both compile and skip without
PROXMOX_TEST_*, and have not run against a host yet. make
test-integration-proxmox runs them. docs/testing.md gains the proxmox,
nocloud and keyedlock rows, the Live Proxmox gaps and the integration
section; spec entry 21's section 11 list is ticked.

Waits that could hang the package under a mutation are bounded: the
concurrent-Create loop and eight direct blocking calls in older tests.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

internal/provider/proxmox/create_test.go

@@ -1196,15 +1196,20 @@ func TestCreate_concurrentSameNameSerialised(t *testing.T) {
 	release := fx.fake.blockOn("Clone")
 	ids := make([]string, 2)
 	errs := make([]error, 2)
+	first := make(chan struct{})
 	var wg sync.WaitGroup
-	for i := range ids {
-		wg.Go(func() { ids[i], errs[i] = fx.p.Create(fx.ctx, fx.req()) })
-		if i == 0 {
-			for fx.fake.callCount("Clone") == 0 {
-				time.Sleep(time.Millisecond)
-			}
+	wg.Go(func() {
+		defer close(first)
+		ids[0], errs[0] = fx.p.Create(fx.ctx, fx.req())
+	})
+	for fx.fake.callCount("Clone") == 0 {
+		select {
+		case <-first:
+			t.Fatalf("first Create returned %v without cloning", errs[0])
+		case <-time.After(time.Millisecond):
 		}
 	}
+	wg.Go(func() { ids[1], errs[1] = fx.p.Create(fx.ctx, fx.req()) })
 	close(release)
 	wg.Wait()
 	for i, err := range errs {
@@ -1233,7 +1238,8 @@ func TestCreate_cancelledMidWaitReturnsPromptly(t *testing.T) {
 		return nil
 	})
 	start := time.Now()
-	_, err := fx.p.Create(ctx, fx.req())
+	var err error
+	within(t, 10*time.Second, func() { _, err = fx.p.Create(ctx, fx.req()) })
 	if elapsed := time.Since(start); elapsed > 5*time.Second {
 		t.Errorf("Create took %v after cancellation", elapsed)
 	}