testing: exploratory UI for followup actions

This adds an API that extensions can use to contribute 'followup'
actions around test messages. Here just a dummy hello world using
copilot, but extensions could have any action here, such as actions to
update snapshots if a test failed:

![](https://memes.peet.io/img/24-05-c1f3e073-a2da-4f16-a033-c8f7e5cd4864.png)

Implemented using a simple provider API.

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

@@ -15,6 +15,11 @@ declare module 'vscode' {
 		 */
 		export function runTests(run: TestRunRequest, token?: CancellationToken): Thenable<void>;
 
+		/**
+		 * Registers a provider that can provide follow-up actions for a test failure.
+		 */
+		export function registerTestFollowupProvider(provider: TestFollowupProvider): Disposable;
+
 		/**
 		 * Returns an observer that watches and can request tests.
 		 */
@@ -31,6 +36,10 @@ declare module 'vscode' {
 		export const onDidChangeTestResults: Event<void>;
 	}
 
+	export interface TestFollowupProvider {
+		provideFollowup(result: TestRunResult, test: TestResultSnapshot, taskIndex: number, messageIndex: number, token: CancellationToken): ProviderResult<Command[]>;
+	}
+
 	export interface TestObserver {
 		/**
 		 * List of tests returned by test provider for files in the workspace.