Fix issues with agent mode disabled (microsoft#258371)

- When agent mode is selected, then disabled, fall back to ask mode
- Disable custom modes when agent mode disabled
Fix microsoft/vscode-copilot#19072

Author: roblourens

src/vs/workbench/contrib/chat/browser/chatInputPart.ts

@@ -693,7 +693,7 @@ export class ChatInputPart extends Disposable implements IHistoryNavigationWidge
 		const currentMode = this._currentModeObservable.get();
 		const validMode = this.chatModeService.findModeById(currentMode.id);
 		if (!validMode) {
-			this.setChatMode2(ChatMode.Agent);
+			this.setChatMode(ChatModeKind.Agent);
 			return;
 		}
 	}

src/vs/workbench/contrib/chat/common/chatModes.ts

@@ -160,7 +160,12 @@ export class ChatModeService extends Disposable implements IChatModeService {
 	}
 
 	getModes(): { builtin: readonly IChatMode[]; custom: readonly IChatMode[] } {
-		return { builtin: this.getBuiltinModes(), custom: Array.from(this._customModeInstances.values()) };
+		return {
+			builtin: this.getBuiltinModes(),
+			custom: this.chatAgentService.hasToolsAgent ?
+				Array.from(this._customModeInstances.values()) :
+				[]
+		};
 	}
 
 	private getFlatModes(): IChatMode[] {