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/src/api-sections.ts

@@ -172,8 +172,6 @@ export function hasRequestSection(source: string): boolean {
   return scanTopLevelBlocks(source, SECTION_NAMES).has("request");
 }
 
-const emptySections: ApiSections = { parameters: [], body: [], response: "", error: "" };
-
 /** Shape of an entry parsed from an `apis { }` container body. */
 export interface ApiEntry {
   mode: "any";
@@ -206,6 +204,12 @@ export function parseApiEntries(source: string): ApiEntry[] {
     }
     const path = lx.readPath();
     const entryBody = lx.readBalancedBraces();
+    const sections = parseApiSections(entryBody);
+    if (sections === null) {
+      throw new LexError(
+        `Api entry "${nameToken.value}" has a bare body; declare a "response { }" section instead`,
+      );
+    }
 
     entries.push({
       mode: "any",
@@ -213,7 +217,7 @@ export function parseApiEntries(source: string): ApiEntry[] {
       method: methodToken.value.toUpperCase(),
       path,
       body: "",
-      sections: parseApiSections(entryBody) ?? emptySections,
+      sections,
     });
   }