Add compact JSON formatter with format-on-paste (§03A)

src/core/json-format.test.ts

@@ -0,0 +1,45 @@
+import { describe, expect, it } from 'vitest';
+import { formatJson } from './json-format';
+
+describe('formatJson', () => {
+  it('pretty-prints a valid object with the default indent', () => {
+    expect(formatJson('{"a":1,"b":2}')).toBe('{"a": 1, "b": 2}');
+  });
+
+  it('keeps short arrays compact (Vega house style)', () => {
+    expect(formatJson('{"values":[1,2,3]}')).toBe('{"values": [1, 2, 3]}');
+  });
+
+  it('wraps content that exceeds the line-length budget', () => {
+    expect(formatJson('{"a":1,"b":2,"c":3}', { maxLength: 5 })).toBe(
+      '{\n  "a": 1,\n  "b": 2,\n  "c": 3\n}',
+    );
+  });
+
+  it('respects a custom indent width', () => {
+    const out = formatJson('{"a":1,"b":2}', { indent: 4, maxLength: 5 });
+    expect(out).toBe('{\n    "a": 1,\n    "b": 2\n}');
+  });
+
+  it('returns null for invalid JSON so callers can skip', () => {
+    expect(formatJson('{"a":')).toBeNull();
+    expect(formatJson('not json')).toBeNull();
+  });
+
+  it('returns null for empty or whitespace-only input', () => {
+    expect(formatJson('')).toBeNull();
+    expect(formatJson('   \n  ')).toBeNull();
+  });
+
+  it('is idempotent — formatting formatted text changes nothing', () => {
+    const once = formatJson('{"a":1,"b":[1,2,3],"c":{"d":4}}');
+    expect(once).not.toBeNull();
+    expect(formatJson(once!)).toBe(once);
+  });
+
+  it('handles top-level arrays and primitives', () => {
+    expect(formatJson('[1,2,3]')).toBe('[1, 2, 3]');
+    expect(formatJson('42')).toBe('42');
+    expect(formatJson('true')).toBe('true');
+  });
+});