fix(json-columns): skip JSON select columns with no model scan target

Requesting a JSON sub-field (e.g. jsonvalue->'product'->>'cost') that has
no matching bun scanonly field on the model made the whole read fail with
"bun: ModelX does not have column Y", since bun scans SELECT results
straight into the typed model struct.

Add reflection.HasColumn to check whether the model can actually receive
a given column (including scanonly fields, walking embedded structs), and
gate the JSON select-column expression on it in ApplySelectColumns
(shared by websocketspec/mqttspec) and the resolvespec/restheadspec
handlers. When there's no scan target, drop just that column with a
warning instead of erroring the whole request.

pkg/reflection/model_utils_test.go

@@ -411,6 +411,23 @@ func TestGetModelColumnsWithEmbedded(t *testing.T) {
 	}
 }
 
+func TestHasColumn(t *testing.T) {
+	m := ModelWithEmbedded{}
+
+	for _, col := range []string{"name", "description", "rid_base", "created_at", "cql1", "cql2"} {
+		if !HasColumn(m, col) {
+			t.Errorf("HasColumn(%q) = false, want true", col)
+		}
+	}
+
+	if HasColumn(m, "nonexistent_column") {
+		t.Error("HasColumn(nonexistent_column) = true, want false")
+	}
+	if HasColumn(m, "") {
+		t.Error("HasColumn(\"\") = true, want false")
+	}
+}
+
 func TestIsColumnWritableWithEmbedded(t *testing.T) {
 	tests := []struct {
 		name       string