Fix negative caret width rendering multi-line expression errors (#1793)

The expression preamble length was subtracted from endColumn even
when endColumn came from the already-stripped sourceLine.length(),
so multi-line frames got a negative caret width and String.repeat
threw. Apply the offset only to the frame-derived columns.

---------

Co-authored-by: Dan Chao <dan.chao@apple.com>

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

@@ -128,17 +128,13 @@ public final class StackTraceRenderer {
       sourceLine = sourceLine.substring(PREAMBLE_LENGTH);
       hasPreamble = true;
     }
-    var startColumn = frame.getStartColumn() - leadingWhitespace;
+    var preambleOffset = hasPreamble ? PREAMBLE_LENGTH : 0;
+    var startColumn = frame.getStartColumn() - leadingWhitespace - preambleOffset;
     var endColumn =
         frame.getStartLine() == frame.getEndLine()
-            ? frame.getEndColumn() - leadingWhitespace
+            ? frame.getEndColumn() - leadingWhitespace - preambleOffset
             : sourceLine.length();
 
-    if (hasPreamble) {
-      startColumn -= PREAMBLE_LENGTH;
-      endColumn -= PREAMBLE_LENGTH;
-    }
-
     var prefix = frame.getStartLine() + " | ";
     out.append(AnsiTheme.STACK_TRACE_MARGIN, leftMargin)
         .append(AnsiTheme.STACK_TRACE_LINE_NUMBER, prefix);

pkl-core/src/test/kotlin/org/pkl/core/EvaluateExpressionTest.kt

@@ -165,6 +165,40 @@ class EvaluateExpressionTest {
       )
   }
 
+  @Test
+  fun `evaluate throwing multi-line expression renders without crashing`() {
+    val error =
+      assertThrows<PklException> {
+        evaluate(
+          "x = 1",
+          """
+          let (_a = 1)
+          let (_b = 2)
+            throw("boom")
+          """
+            .trimIndent(),
+        )
+      }
+
+    assertThat(error.message)
+      .isEqualTo(
+        """
+        –– Pkl Error ––
+        boom
+
+        3 | throw("boom")
+            ^^^^^^^^^^^^^
+        at  (repl:text)
+
+        1 | let (_a = 1)
+            ^^^^^^^^^^^^
+        at repl:text.<let expr> (repl:text)
+
+        """
+          .trimIndent()
+      )
+  }
+
   @Test
   fun `evaluate import`() {
     val result = evaluate("import \"pkl:base\"", "base")