fix: make sure register handler when ipc emitter add listener

[theanarkh (theanarkh)]

Fixes: #324678

When the last listener is disposed, we will remove the handler from the map. However, we will not add the handler back to the map when re-subscribing to the emitter, so the event will not be emitted anymore. It is better register handler in onWillAddFirstListener and delete it in onDidRemoveLastListener.

example.

const emitter = client.listen('hello');
const dispose1 = emitter(() => {);
dispose1.dispose();
// can not receive any messages.
const dispose2 = emitter(() => {);

[Copilot]

Pull request overview

This PR fixes ChannelClient.requestEvent() in src/vs/base/parts/ipc/common/ipc.ts so that re-adding a listener to an IPC event after disposing the previous listener will correctly receive messages again.

Changes:

• Register the IPC response handler in Emitter’s onWillAddFirstListener hook (instead of at event creation time).
• Ensure the handler is re-registered whenever the event transitions from 0 → 1 listeners, matching the existing cleanup on last-listener removal.

[roblourens]

Thanks for the PR, I think this change is good but please file an issue with a better description of the issue, and add unit tests for the scenario

[theanarkh (theanarkh)]

roblourens Thanks ! I have updated the code, please help review again.

[roblourens]

Thanks, can you open an issue?

[theanarkh (theanarkh)]

Thanks, can you open an issue?

Done.