fix(y-partyserver): make onMessage overridable via super

[aryasaatvik]

Problem

When extending YServer to handle custom string messages alongside Yjs binary messages, developers couldn't override onMessage and call super.onMessage() because it was defined as a class field rather than a method.

// This didn't work ❌
class MyYServer extends YServer {
  onMessage(conn, message) {
    if (typeof message === "string") {
      // handle custom JSON
      return;
    }
    super.onMessage(conn, message); // undefined - class fields aren't on prototype
  }
}

Solution

Refactored onMessage from a class field to a proper method, moving the chunking handler to a private handleChunkedMessage field.

// Now works ✓
class MyYServer extends YServer {
  onMessage(conn, message) {
    if (typeof message === "string") {
      const data = JSON.parse(message);
      // handle custom messages
      return;
    }
    super.onMessage(conn, message); // properly delegates to parent
  }
}

This enables mixed message handling (custom JSON + Yjs protocol) without modifying the library internals.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 794ce47

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[threepointone]

actually, how about this as a better first class solution #290