sessions: add multi-chat support to Copilot provider (#preview)

Add support for multiple chats within a single session in the Copilot
Chat sessions provider, gated behind the preview setting
`sessions.github.copilot.multiChatSessions`.

Key changes:
- Integrate SessionsGroupModel into CopilotChatSessionsProvider for
  chat-to-session grouping with storage persistence
- Add _sendSubsequentChat() for creating new chats in existing sessions
  with workspace isolation mode
- _chatToSession() builds multi-chat observable from group model with
  aggregated status, updatedAt, isRead, lastTurnEnd across all chats
- Session title and properties always come from the primary (first) chat
- getSessions() deduplicates by group ID when multi-chat is enabled
- deleteSession() removes all chats in the group
- deleteChat() removes individual chats; delegates to deleteSession
  when only one chat remains
- _refreshSessionCacheMultiChat() handles removed chats that belong to
  groups with siblings as changed events on the parent session
- Management service tracks active chat via autorun on chats observable
- All multi-chat code paths fall back to single-chat behavior when the
  setting is disabled
- Comprehensive unit tests for the provider

Author: sandy081

src/vs/sessions/contrib/copilotChatSessions/browser/copilotChatSessions.contribution.ts

@@ -6,9 +6,24 @@
 import { IWorkbenchContribution, registerWorkbenchContribution2, WorkbenchPhase } from '../../../../workbench/common/contributions.js';
 import { IInstantiationService } from '../../../../platform/instantiation/common/instantiation.js';
 import { Disposable } from '../../../../base/common/lifecycle.js';
-import { CopilotChatSessionsProvider } from '../../copilotChatSessions/browser/copilotChatSessionsProvider.js';
+import { CopilotChatSessionsProvider, COPILOT_MULTI_CHAT_SETTING } from '../../copilotChatSessions/browser/copilotChatSessionsProvider.js';
 import '../../copilotChatSessions/browser/copilotChatSessionsActions.js';
 import { ISessionsProvidersService } from '../../sessions/browser/sessionsProvidersService.js';
+import { Registry } from '../../../../platform/registry/common/platform.js';
+import { IConfigurationRegistry, Extensions as ConfigurationExtensions } from '../../../../platform/configuration/common/configurationRegistry.js';
+import { localize } from '../../../../nls.js';
+
+Registry.as<IConfigurationRegistry>(ConfigurationExtensions.Configuration).registerConfiguration({
+	id: 'sessions',
+	properties: {
+		[COPILOT_MULTI_CHAT_SETTING]: {
+			type: 'boolean',
+			default: false,
+			tags: ['preview'],
+			description: localize('sessions.github.copilot.multiChatSessions', "Whether to enable multiple chats within a single session in the Copilot Chat sessions provider."),
+		},
+	},
+});
 
 /**
  * Registers the {@link CopilotChatSessionsProvider} as a sessions provider.