Fix pkl:test fact power assertions when member source section is unavailable (#1571)

Power assertions only work when the source section is available. If it
is unavailable, power assertions throw a ParserError (unexpected EOF on
an empty input) when re-parsing the expression for presentation.

pkl-core/src/main/java/org/pkl/core/runtime/PowerAssertions.java

@@ -30,6 +30,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.function.Consumer;
 import java.util.stream.Collectors;
+import org.pkl.core.PklBugException;
 import org.pkl.core.ast.ConstantValueNode;
 import org.pkl.core.ast.expression.member.InferParentWithinMethodNode;
 import org.pkl.core.ast.expression.member.InferParentWithinObjectMethodNode;
@@ -83,6 +84,10 @@ public class PowerAssertions {
       SourceSection sourceSection,
       Map<Node, List<Object>> trackedValues,
       @Nullable Consumer<AnsiStringBuilder> firstFrameSuffix) {
+    if (!sourceSection.isAvailable()) {
+      throw new PklBugException("Power assertions require an available source section");
+    }
+
     out.appendSandboxed(
         () -> {
           var lines = lines(sourceSection);

pkl-core/src/main/java/org/pkl/core/runtime/TestRunner.java

@@ -102,7 +102,7 @@ public final class TestRunner {
                 try {
                   var factValue = VmUtils.readMember(listing, idx);
                   if (factValue == Boolean.FALSE) {
-                    if (PowerAssertions.isEnabled()) {
+                    if (PowerAssertions.isEnabled() && member.getSourceSection().isAvailable()) {
                       try (var valueTracker = valueTrackerFactory.create()) {
                         listing.cachedValues.clear();
                         VmUtils.readMember(listing, idx);