fix: resolve code-review findings (locally verified)

Server-054/055/056, Contracts-020/021/022, Tests-036/038/039,
IntegrationTests-030/031/032 (+033 deferred to live rig),
Client.Dotnet-026/028/029 (+027 won't-fix), Client.Go-030..034,
Client.Python-032..036, Client.Rust-033..038.

Key fix: SessionEventDistributor orphaned a subscriber that registered after
the pump completed but before disposal (Server-056) -> register paths now
complete late registrants under _lifecycleLock; regression test added. The
racy dashboard-mirror gRPC test made deterministic (Tests-039).

Verified green locally: gateway Tests targeted classes (GatewaySession,
SessionEventDistributor, GatewayOptionsValidator, ProtobufContractRoundTrip,
GatewaySessionDashboardMirror) + dotnet/go/python/rust client suites.

clients/go/cmd/mxgw-go/main_test.go

@@ -537,3 +537,43 @@ func TestRunBatchHandlesLongCommandLine(t *testing.T) {
 		t.Fatalf("EOR sentinel count = %d, want 2 (one per command, even when first is too long); out length = %d", count, len(out))
 	}
 }
+
+// TestRunBenchReadBulkRejectsNonPositiveDuration pins the -duration-seconds
+// positivity guard so the bench window cannot be configured to zero/negative.
+func TestRunBenchReadBulkRejectsNonPositiveDuration(t *testing.T) {
+	var stdout, stderr bytes.Buffer
+	err := runWithIO(t.Context(), []string{"bench-read-bulk", "-duration-seconds", "0"}, &stdout, &stderr)
+	if err == nil || !strings.Contains(err.Error(), "duration-seconds must be positive") {
+		t.Fatalf("bench-read-bulk -duration-seconds 0 error = %v", err)
+	}
+}
+
+// TestRunStreamEventsRequiresSessionID pins the session-id guard so stream-events
+// fails fast before dialing when no session id is supplied.
+func TestRunStreamEventsRequiresSessionID(t *testing.T) {
+	var stdout, stderr bytes.Buffer
+	err := runWithIO(t.Context(), []string{"stream-events", "-plaintext", "-api-key", "test"}, &stdout, &stderr)
+	if err == nil || !strings.Contains(err.Error(), "session-id is required") {
+		t.Fatalf("stream-events without -session-id error = %v", err)
+	}
+}
+
+// TestRunWriteBulkVariantRejectsMismatchedHandlesAndValues pins the len-mismatch
+// guard so a write-bulk with unequal item-handles / values counts fails fast
+// before any dial.
+func TestRunWriteBulkVariantRejectsMismatchedHandlesAndValues(t *testing.T) {
+	var stdout, stderr bytes.Buffer
+	err := runWithIO(t.Context(), []string{
+		"write-bulk",
+		"-session-id", "s1",
+		"-server-handle", "1",
+		"-item-handles", "1,2",
+		"-values", "10",
+		"-type", "int32",
+		"-plaintext",
+		"-api-key", "test",
+	}, &stdout, &stderr)
+	if err == nil || !strings.Contains(err.Error(), "does not match values count") {
+		t.Fatalf("write-bulk mismatched handles/values error = %v", err)
+	}
+}