Fix: Raise WebSocketDisconnect instead of RuntimeError on closed conn…

[Manas-sin]

Summary

Replaces the generic RuntimeError with WebSocketDisconnect (code 1006) when attempting to send or receive messages on a WebSocket connection that has already been closed.

This improves error handling by allowing applications to catch a specific exception type instead of relying on string matching for RuntimeError.

Fixes #2767.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly. (No doc changes needed for this internal fix, but tests are updated)

[Kludex]

But this doesn't make sense; the client isn't supposed to see those error messages.

Let's continue the discussion on the other PR.