fix(syntax): reject bare apis-container bodies and cross-mode duplicate api names

Bare bodies inside apis {} silently discarded their text with no error,
producing a do-nothing block. They now throw a ParseError naming the entry
and pointing at the response {} section. Duplicate-name detection for
dataApis moved from an incremental, order-dependent check (only saw prior
entries in the array) to a single post-parse pass over the whole ast.dataApis,
so it catches cross-mode duplicates (apis {} vs ssr { api }) regardless of
declaration order.

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

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

@@ -69,3 +69,47 @@ test("duplicate names inside one container are rejected", () => {
     ),
   ).toThrow(/duplicate/i);
 });
+
+test("a bare body inside apis {} is a parse error naming response", () => {
+  expect(() => parse(page(`    bare GET /api/z { return data }`))).toThrow(/response/i);
+});
+
+test("apis entry followed by an ssr api of the same name is rejected", () => {
+  const src = `page Repro {
+  apis {
+    foo GET /api/foo { response { return data } }
+  }
+
+  ssr {
+    api foo GET /api/foo { return data }
+  }
+
+  view { <main>x</main> }
+}
+`;
+  expect(() => parse(src)).toThrow(/duplicate/i);
+});
+
+test("ssr api followed by an apis entry of the same name is rejected", () => {
+  const src = `page Repro {
+  ssr {
+    api foo GET /api/foo { return data }
+  }
+
+  apis {
+    foo GET /api/foo { response { return data } }
+  }
+
+  view { <main>x</main> }
+}
+`;
+  expect(() => parse(src)).toThrow(/duplicate/i);
+});
+
+test("an explicit empty response section is accepted, unlike a bare body", () => {
+  const ast = parse(page(`    empty GET /api/e { response { } }`));
+
+  const block = ast.dataApis[0]!;
+  expect(block.name).toBe("empty");
+  expect(block.sections?.response.trim()).toBe("");
+});