Simplify bounded HTML reads

The bounded HTML path can rely on the standard Response body type instead
of defensive assertions for non-standard mocks.  The parser now preserves
HTML-labelled JSON compatibility by attempting JSON.parse() directly and
converts syntax failures into the same FetchError used for ordinary HTML.

#913 (comment)
#913 (comment)
#913 (comment)
#913 (comment)
#913 (comment)

Assisted-by: Codex:gpt-5.5

Author: dahlia

packages/vocab-runtime/src/docloader.test.ts

@@ -369,38 +369,6 @@ test("getDocumentLoader()", async (t) => {
     deepStrictEqual(canceled, true);
   });
 
-  await t.test("HTML body without getReader falls back to text", async () => {
-    const response = new Response(
-      JSON.stringify({
-        "@context": "https://www.w3.org/ns/activitystreams",
-        id: "https://example.com/body-without-get-reader",
-        name: "Fetched object",
-        type: "Object",
-      }),
-      { headers: { "Content-Type": "text/html; charset=utf-8" } },
-    );
-    Object.defineProperty(response, "body", { value: {} });
-    deepStrictEqual(
-      await getRemoteDocument(
-        "https://example.com/body-without-get-reader",
-        response,
-        () => {
-          throw new Error("unexpected alternate fetch");
-        },
-      ),
-      {
-        contextUrl: null,
-        documentUrl: "https://example.com/body-without-get-reader",
-        document: {
-          "@context": "https://www.w3.org/ns/activitystreams",
-          id: "https://example.com/body-without-get-reader",
-          name: "Fetched object",
-          type: "Object",
-        },
-      },
-    );
-  });
-
   fetchMock.get("https://example.com/404", { status: 404 });
 
   await t.test("not ok", async () => {

packages/vocab-runtime/src/docloader.ts

@@ -122,9 +122,8 @@ function createResponseMetadata(response: Response): Response {
 }
 
 async function cancelResponseBody(response: Response): Promise<void> {
-  const body = response.body as { cancel?: unknown } | null;
-  if (body != null && typeof body.cancel === "function") {
-    await body.cancel();
+  if (response.body != null) {
+    await response.body.cancel();
   }
 }
 
@@ -135,28 +134,15 @@ async function readBoundedText(
   const contentLength = response.headers.get("Content-Length");
   if (contentLength != null) {
     const size = Number(contentLength);
-    if (Number.isFinite(size) && size > maxBytes) {
+    if (size > maxBytes) {
       await cancelResponseBody(response);
       return { text: "", size, tooLarge: true };
     }
   }
 
   if (response.body == null) return { text: "", size: 0, tooLarge: false };
 
-  const body = response.body as ReadableStream<Uint8Array> & {
-    getReader?: unknown;
-  };
-  if (typeof body.getReader !== "function") {
-    const text = await response.text();
-    const size = new TextEncoder().encode(text).byteLength;
-    return {
-      text: size <= maxBytes ? text : "",
-      size,
-      tooLarge: size > maxBytes,
-    };
-  }
-
-  const reader = body.getReader();
+  const reader = response.body.getReader();
   const decoder = new TextDecoder();
   let text = "";
   let size = 0;
@@ -319,10 +305,10 @@ export async function getRemoteDocument(
           return await fetch(new URL(attribs.href, docUrl).href);
         }
       }
-      const trimmed = html.text.trimStart();
-      if (trimmed.startsWith("{") || trimmed.startsWith("[")) {
+      try {
         document = JSON.parse(html.text);
-      } else {
+      } catch (error) {
+        if (!(error instanceof SyntaxError)) throw error;
         throw new FetchError(
           documentUrl,
           `HTML document has no ActivityPub alternate link ` +