Don't throw in setRequestTools when agent is gone (#313068)

Fixes #312803

The autorun in chatServiceImpl that calls setRequestTools is per-request
and can fire after the target agent has been unregistered (e.g.,
setup.vscode is disposed when Copilot setup completes while a request
is still in-flight). Throwing here surfaces as an unhandled error from
inside the autorun. Match the existing pattern used by setYieldRequested
and getFollowups in the same file and silently return.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/vs/workbench/contrib/chat/common/participants/chatAgents.ts

@@ -537,7 +537,7 @@ export class ChatAgentService extends Disposable implements IChatAgentService {
 	setRequestTools(id: string, requestId: string, tools: UserSelectedTools): void {
 		const data = this._agents.get(id);
 		if (!data?.impl) {
-			throw new Error(`No activated agent with id "${id}"`);
+			return;
 		}
 
 		data.impl.setRequestTools?.(requestId, tools);