fix(syntax): make api-section scanning string/comment-aware

Reuse tokenizer.readBalancedBraces string/comment skipping (extracted as
skipLiteralOrComment) for both section detection and slicing, instead of a
second hand-rolled brace counter. Fixes truncation on braces inside strings
and false-positive sectioned detection from keywords inside comments/strings.
Also switch api-sections.ts errors from plain Error to LexError so parser.ts
upgrades them to ParseError with an offset.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/syntax/test/api-block.test.ts

@@ -96,3 +96,42 @@ test("request inside an ssr block is rejected with a message naming the restrict
 
   expect(() => parse(source)).toThrow(/ssr[\s\S]*request/i);
 });
+
+test("a brace inside a string literal in the response body does not truncate the section", () => {
+  const ast = parse(
+    page(`    api searchUsers POST /api/users {
+      request {
+        body {
+          name?: string
+        }
+      }
+
+      response {
+        return "a } weird string"
+      }
+
+      error {
+        return []
+      }
+    }`),
+  );
+
+  const block = ast.dataApis[0]!;
+  expect(block.sections?.response.trim()).toBe('return "a } weird string"');
+  expect(block.sections?.error.trim()).toBe("return []");
+});
+
+test("a legacy block whose comment or string mentions a section keyword stays legacy", () => {
+  const ast = parse(
+    page(`    api legacyUsers GET /api/users {
+      // fall back to a manual request { } if this fails
+      return "response { not a section }"
+    }`),
+  );
+
+  const block = ast.dataApis[0]!;
+  expect(block.sections).toBeUndefined();
+  expect(block.body.trim()).toBe(
+    '// fall back to a manual request { } if this fails\n      return "response { not a section }"',
+  );
+});