ChatRequestHandler should allow returning void (microsoft#208349)

Fix microsoft#205835

Author: roblourens

extensions/vscode-api-tests/src/singlefolder-tests/chat.test.ts

@@ -42,7 +42,6 @@ suite('chat', () => {
 
 		const participant = chat.createChatParticipant('api-test.participant', (request, context, _progress, _token) => {
 			emitter.fire({ request, context });
-			return null;
 		});
 		participant.isDefault = true;
 		disposables.push(participant);

src/vs/workbench/api/common/extHostChatAgents2.ts

@@ -620,7 +620,7 @@ class ExtHostChatAgent {
 		} satisfies vscode.ChatParticipant;
 	}
 
-	invoke(request: vscode.ChatRequest, context: vscode.ChatContext, response: vscode.ChatExtendedResponseStream, token: CancellationToken): vscode.ProviderResult<vscode.ChatResult> {
+	invoke(request: vscode.ChatRequest, context: vscode.ChatContext, response: vscode.ChatExtendedResponseStream, token: CancellationToken): vscode.ProviderResult<vscode.ChatResult | void> {
 		return this._requestHandler(request, context, response, token);
 	}
 }

src/vscode-dts/vscode.proposed.chatParticipant.d.ts

@@ -184,7 +184,7 @@ declare module 'vscode' {
 	/**
 	 * A chat request handler is a callback that will be invoked when a request is made to a chat participant.
 	 */
-	export type ChatRequestHandler = (request: ChatRequest, context: ChatContext, response: ChatResponseStream, token: CancellationToken) => ProviderResult<ChatResult>;
+	export type ChatRequestHandler = (request: ChatRequest, context: ChatContext, response: ChatResponseStream, token: CancellationToken) => ProviderResult<ChatResult | void>;
 
 	/**
 	 * A chat participant can be invoked by the user in a chat session, using the `@` prefix. When it is invoked, it handles the chat request and is solely

src/vscode-dts/vscode.proposed.chatParticipantAdditions.d.ts

@@ -167,7 +167,7 @@ declare module 'vscode' {
 		constructor(label: string | CompletionItemLabel, values: ChatVariableValue[]);
 	}
 
-	export type ChatExtendedRequestHandler = (request: ChatRequest, context: ChatContext, response: ChatExtendedResponseStream, token: CancellationToken) => ProviderResult<ChatResult>;
+	export type ChatExtendedRequestHandler = (request: ChatRequest, context: ChatContext, response: ChatExtendedResponseStream, token: CancellationToken) => ProviderResult<ChatResult | void>;
 
 	export namespace chat {
 		/**