fix(client): handle empty JSON response body without content-length

[LeSingh1]

Problem

defaultParseResponse (src/internal/parse.ts) already special-cases an empty JSON response body:

const contentLength = response.headers.get('content-length');
if (contentLength === '0') {
  // if there is no content we can't do anything
  return undefined as T;
}

const json = await response.json();

That guard only fires when the server sends an explicit content-length: 0 header. When the header is absent — which is common over HTTP/2, with chunked transfer encoding, or behind gateways/proxies and OpenAI-compatible servers — an empty body with a JSON content-type falls through to response.json() and throws:

SyntaxError: Unexpected end of JSON input

So two byte-for-byte identical empty responses parse differently depending solely on whether the server happened to include content-length: 0. The existing guard shows this case is meant to be handled gracefully; it's just incomplete.

Reproduction

const client = new OpenAI({
  apiKey: '...',
  // An empty body with a JSON content-type and no `content-length` header.
  // (`new Response('')` in undici sets no content-length.)
  fetch: async () => new Response('', { headers: { 'Content-Type': 'application/json' } }),
});

await client.request({ path: '/foo', method: 'get' });
// Before: throws `SyntaxError: Unexpected end of JSON input`
// After:  resolves to `undefined`

Fix

Read the body as text first and treat an empty payload the same as an explicit content-length: 0. Non-empty bodies are still parsed with JSON.parse, so malformed JSON continues to throw as before.

const bodyText = await response.text();
if (!bodyText) {
  return undefined as T;
}
const json = JSON.parse(bodyText);

Verification

Added a response parsing test group in tests/index.test.ts covering:

• empty JSON body without content-length → resolves to undefined (the fixed case; fails on main with SyntaxError: Unexpected end of JSON input)
• empty JSON body with content-length: 0 → resolves to undefined (existing behavior preserved)
• non-empty JSON body → parsed correctly
• malformed non-empty JSON body → still throws

$ npx jest tests/index.test.ts
Tests:       68 passed, 68 total

npx tsc --noEmit, eslint, and prettier --check all pass on the changed files.