Chat does not register agent again when extension re-installed

[bpasero]

Similar to #250381

Steps to Reproduce:

1. you can test this running out of sources, use a fresh user data dir
2. type "hi" into chat and run through setup
3. uninstall copilot extension and click to restart extension host
4. type "hi" again, extension installs again automatically

=> 🐛 notice the warning that copilot failed to get ready

This fails here:

vscode/src/vs/workbench/contrib/chat/common/chatServiceImpl.ts

Lines 499 to 502 in 3a7f042

	const defaultAgent = this.chatAgentService.getActivatedAgents().find(agent => agent.id === defaultAgentData.id);
	if (!defaultAgent) {
	throw new ErrorNoTelemetry('No default agent registered');
	}

@roblourens so it looks like the agent only registers once?

Interestingly when I retry, it just works then 🤔

[bpasero]

The underlying reason is that we end up in this cached activation:

vscode/src/vs/workbench/services/extensions/common/extensionHostManager.ts

Line 331 in 3ba947e

return this._cachedActivationEvents.get(activationEvent)!;

And thus we seem to be returning too early here:

vscode/src/vs/workbench/contrib/chat/common/chatServiceImpl.ts

Lines 499 to 502 in 3ba947e

	const defaultAgent = this.chatAgentService.getActivatedAgents().find(agent => agent.id === defaultAgentData.id);
	if (!defaultAgent) {
	throw new ErrorNoTelemetry('No default agent registered');
	}