fix: fix MessageLocation.end being corrupted (wrong type) for YAML parse errors

We returned a plain object that stood in for a SourcePosition, but
MessageLocation does instanceof SourcePosition checks. Also added a
safeguard to the constructor now.

Author: Yogu

src/model/validation/location.ts

@@ -46,6 +46,13 @@ export class MessageLocation {
                 value: new ProjectSource(this.sourceName, ''),
             });
         }
+
+        if (typeof _start !== 'number' && !(_start instanceof SourcePosition)) {
+            throw new Error(`start must be either a number or a SourcePosition`);
+        }
+        if (typeof _end !== 'number' && !(_end instanceof SourcePosition)) {
+            throw new Error(`end must be either a number or a SourcePosition`);
+        }
     }
 
     static from(location: LocationLike | undefined): MessageLocation | undefined {

src/schema/schema-utils.ts

@@ -256,5 +256,5 @@ export function getLineEndPosition(targetLine: number, source: ProjectSource): S
         }
     }
 
-    return { line: targetLine, offset: curIndex - 1, column: column };
+    return new SourcePosition(curIndex - 1, targetLine, column);
 }