Agent sessions: support multi-select for mark read/unread and archive/unarchive

[Copilot]

Enables multi-select support in the agent sessions tree view, allowing batch operations on multiple sessions at once.

Changes

• Tree control: Set multipleSelectionSupport: true to enable standard multi-select gestures (Ctrl/Cmd+click, Shift+click)
• Context handling: Extended IMarshalledAgentSessionContext with sessions array; context menu now passes all selected sessions when multiple are selected
• Actions: Refactored BaseAgentSessionAction from runWithSession to runWithSessions to iterate over all selected sessions for:
  • Mark as Read / Unread
  • Archive / Unarchive
• Consolidated confirmation dialog: When archiving multiple sessions with pending edits, shows a single confirmation dialog that displays the count of affected sessions and provides "Keep & Archive" or "Undo & Archive" options, rather than showing separate dialogs for each session
• Backward compatibility: Uses context.sessions ?? [context.session] fallback for callers that don't provide the new field

Selection behavior

Follows standard file explorer pattern: right-clicking a session in the current selection operates on all selected sessions; right-clicking outside the selection operates on only that session.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Agent sessions: support multi select for operations like marking read or archive</issue_title>
<issue_description>This would make it much easier to cleanup many sessions from the view.</issue_description>

<agent_instructions>start with allowing multi select for marking read, marking unread, archiving and unarchiving.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Agent sessions: support multi select for operations like marking read or archive #288448

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[Copilot]

Pull request overview

This PR enables multi-select support for the agent sessions tree view, allowing users to perform batch operations on multiple sessions simultaneously. The implementation follows standard file explorer multi-select patterns with Ctrl/Cmd+click and Shift+click gestures.

Changes:

• Enabled multi-select support in the tree control and extended context handling to pass selected sessions
• Refactored mark read/unread and archive/unarchive actions to iterate over multiple sessions
• Added context key tracking for multi-selection state and appropriate preconditions for single-session operations

Reviewed changes

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

File	Description
src/vs/workbench/contrib/chat/common/actions/chatContextKeys.ts	Adds new context key to track when multiple agent sessions are selected
src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionsModel.ts	Extends IMarshalledAgentSessionContext with sessions array for multi-selection support; includes label splitting defensive fix
src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionsControl.ts	Enables multipleSelectionSupport in tree config and updates context menu to pass all selected sessions
src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionsActions.ts	Refactors BaseAgentSessionAction from runWithSession to runWithSessions to handle batch operations; adds precondition to rename action

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionsModel.ts:170

• The sessions field should be marked as optional with a question mark since the code handles its absence with a fallback (context.sessions ?? [context.session]). This would make the TypeScript type definition match the runtime behavior and provide better type safety.

	readonly sessions: IAgentSession[]; // support for multi-selection