fix: invalid JSON output when --json flag used (#2666)

Author: comatory

cli/src/json-check-schema-output-builder.ts

@@ -199,13 +199,23 @@ export class JsonCheckSchemaOutputBuilder {
     return this.data;
   }
 
+  /**
+   * Writes valid JSON either to stdout or a file.
+   */
   async write(): Promise<void> {
-    const output = this.build();
-
     if (this.outFile) {
-      await writeFile(this.outFile, JSON.stringify(output, null, 2));
+      await writeFile(this.outFile, this.serializeOutput(true));
     } else {
-      console.log(output);
+      console.log(this.serializeOutput());
+    }
+  }
+
+  private serializeOutput(formatted = false): string {
+    try {
+      return JSON.stringify(this.build(), null, formatted ? 2 : 0);
+    } catch (err) {
+      console.error('Failed to serialize JSON data.');
+      throw err;
     }
   }
 }

cli/test/json-check-schema-output-builder.test.ts

@@ -297,7 +297,19 @@ describe('JsonCheckSchemaOutputBuilder', () => {
       const b = new JsonCheckSchemaOutputBuilder(EnumStatusCode.OK, 10);
       b.setStatus(true);
       await b.write();
-      expect(spy).toHaveBeenCalledWith(b.build());
+      expect(spy).toHaveBeenCalledWith('{"status":"success","code":0,"rowLimit":10}');
+    });
+
+    it('serializes LintIssue instances to plain objects in JSON output', async () => {
+      const spy = vi.spyOn(console, 'log').mockImplementation(() => {});
+      const b = new JsonCheckSchemaOutputBuilder(EnumStatusCode.OK, 10);
+      b.addLintErrors([new LintIssue({ message: 'lint error', lintRuleType: 'RULE_A' })]);
+      b.addLintWarnings([new LintIssue({ message: 'lint warn', lintRuleType: 'RULE_B' })]);
+      await b.write();
+
+      expect(spy).toHaveBeenCalledWith(
+        '{"status":"error","code":0,"rowLimit":10,"lint":{"errors":[{"lintRuleType":"RULE_A","severity":"warn","message":"lint error"}],"warnings":[{"lintRuleType":"RULE_B","severity":"warn","message":"lint warn"}]}}',
+      );
     });
 
     it('writes JSON to file when outFile provided', async () => {