fix(router): write proper line endings and header for multipart

[df-wg]

Motivation and Context

Users noted that mutlipart subscriptions weren't working with Apollo Client. This PR fixes two issues with the current multipart subscription, in order to fix it:
*. It changes the main Content-Type header to multipart/mixed; boundary=graphql, to fit with the W3 RFC requirements (and the client's explicit parsing)

• It changes the line endings before the message to be /r/n instead of just /n, to fit with the RFC's explicit use of CRLF.

Using it with a client:

const { ApolloClient, InMemoryCache, HttpLink} = require('@apollo/client');
const gql = require('graphql-tag');

// Define your subscription query
const SUBSCRIPTION_QUERY = `
  subscription Test {
    countFor(count: 5)
  }
`;

// HTTP link for queries and mutations
const httpLink = new HttpLink({
  uri: 'http://localhost:3002/graphql',
  headers: {
    Accept: 'multipart/mixed;subscriptionSpec="1.0", application/json',
  },
});

// Create the Apollo Client
const client = new ApolloClient({
  link: httpLink,
  cache: new InMemoryCache(),
});

// Execute the subscription
const testSubscription = () => {
  const observable = client.subscribe({
    query: gql(SUBSCRIPTION_QUERY),
  });

  // Listen for incoming data
  observable.subscribe({
    next(data) {
      console.log('Subscription data:', data);
    },
    error(err) {
      if (err?.message === 'internal system error') {
        console.log("system error, ignore")
      } else {
        console.error('Subscription error:', err);
      }
    },
    complete() {
      console.log('Subscription completed');
    },
  });
};

// Start the test
console.log("hello world")
testSubscription();

prints:

hello world
Subscription data: { data: { countFor: 0 } }
Subscription data: { data: { countFor: 1 } }
Subscription data: { data: { countFor: 2 } }
Subscription data: { data: { countFor: 3 } }
Subscription data: { data: { countFor: 4 } }
Subscription data: { data: { countFor: 5 } }
Subscription completed

Using with curl

❯ curl -i -H "Accept: multipart/mixed" -X POST http://localhost:3002/graphql -d '{"query": "subscription { countFor(count: 2) }"}'
HTTP/1.1 200 OK
Cache-Control: no-cache
Connection: keep-alive
Content-Type: multipart/mixed; boundary=graphql
Vary: Accept-Encoding
X-Accel-Buffering: no
Date: Mon, 20 Jan 2025 07:30:54 GMT
Transfer-Encoding: chunked


--graphql
Content-Type: application/json

{"payload":{"data":{"countFor":0}}}

--graphql
Content-Type: application/json

{"payload":{"data":{"countFor":1}}}

--graphql
Content-Type: application/json

{"payload":{"data":{"countFor":2}}}

--graphql
Content-Type: application/json

{"payload":{"errors":[{"message":"internal system error"}],"data":null}}
--graphql--

Checklist

• I have discussed my proposed changes in an issue and have received approval to proceed.
• I have followed the coding standards of the project.
• Tests or benchmarks have been added or updated.
• Documentation has been updated on https://github.com/wundergraph/cosmo-docs.
• I have read the Contributors Guide.

[github-actions]

Router image scan passed

✅ No security vulnerabilities found in image:

ghcr.io/wundergraph/cosmo/router:sha-afd7e4cd94e296fae7b7cac52751223317f66179