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

[sandy081]

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

[Copilot]

Pull request overview

This PR adds preview-gated support for multiple chats within a single session in the Copilot sessions provider (sessions.github.copilot.multiChatSessions) by introducing a persisted grouping model and updating session lifecycle behaviors to treat a “session” as a group of chats.

Changes:

• Adds a persisted SessionsGroupModel to group Copilot chats into multi-chat sessions and surfaces grouped sessions via aggregated observables (status, updatedAt, isRead, lastTurnEnd).
• Extends the Copilot provider with “subsequent chat” creation/deletion flows while maintaining single-chat behavior when the setting is disabled.
• Adds unit tests for the Copilot sessions provider and updates sessions management to track the active chat during send-and-create flows.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
src/vs/sessions/contrib/sessions/browser/sessionsManagementService.ts	Updates active-chat handling during sendAndCreateChat to react to chat list changes while sending.
src/vs/sessions/contrib/copilotChatSessions/browser/copilotChatSessionsProvider.ts	Implements multi-chat session grouping, subsequent chat creation, grouped deletion, and multi-chat change event handling behind a setting gate.
src/vs/sessions/contrib/copilotChatSessions/browser/copilotChatSessions.contribution.ts	Registers the preview setting controlling multi-chat support for the Copilot provider.
src/vs/sessions/contrib/copilotChatSessions/test/browser/copilotChatSessionsProvider.test.ts	Adds unit tests covering provider identity, capabilities, session listing, and deletion behaviors in single- and multi-chat modes.