Don't return providers in IChatSessionsService methods

[mjbvz]

Further trying to clean up interface so that IChatSessionsService consumers don't know about providers directly

[Copilot]

Pull request overview

This PR cleans up the IChatSessionsService interface by changing activateChatSessionItemProvider to no longer expose providers directly to consumers. The method's return type is changed from Promise<IChatSessionItemProvider | undefined> to Promise<void>.

Changes:

• Modified the IChatSessionsService interface to change activateChatSessionItemProvider return type to Promise<void>
• Created a new private doActivateChatSessionItemProvider method that returns the provider for internal use
• Updated the mock implementation to match the new interface signature

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/vs/workbench/contrib/chat/common/chatSessionsService.ts	Updated interface signature for activateChatSessionItemProvider to return Promise<void>
src/vs/workbench/contrib/chat/browser/chatSessions/chatSessions.contribution.ts	Introduced private doActivateChatSessionItemProvider method for internal use and updated the public method to delegate without returning the provider
src/vs/workbench/contrib/chat/test/common/mockChatSessionsService.ts	Updated mock implementation to match new interface signature with noop implementation