Fix session resource aliasing in chat sessions

[TylerLeonhardt]

Implement a mechanism to register aliases for session resources, allowing session-option lookups to redirect to the correct canonical resource. This change enhances the handling of chat sessions by ensuring that the correct resources are used in session management.

I think this is pretty fragile, but it fixes the critical issues that need to be fixed this release.

ref #298378

[TylerLeonhardt]

FYI @mjbvz

[Copilot]

Pull request overview

This PR adds session-resource aliasing to the chat sessions infrastructure so that when an untitled session is later resolved to a “real” session resource, session-option lookups continue to work against the canonical resource stored in the internal session map.

Changes:

• Extend IChatSessionsService with registerSessionResourceAlias(untitledResource, realResource) for redirecting option lookups.
• Implement alias resolution in ChatSessionsService for session-option getters/setters and option-change notifications.
• Update MainThreadChatAgents2 to register an alias when an untitled session is resolved via newChatSessionItemHandler and update the model’s contributed session resource.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/vs/workbench/contrib/chat/test/common/mockChatSessionsService.ts	Adds the new interface method as a no-op in the mock implementation.
src/vs/workbench/contrib/chat/common/chatSessionsService.ts	Adds the alias-registration API to IChatSessionsService.
src/vs/workbench/contrib/chat/browser/chatSessions/chatSessions.contribution.ts	Implements alias mapping and resolves option lookups + option-change events through the alias map.
src/vs/workbench/api/browser/mainThreadChatAgents2.ts	Registers aliases when resolving untitled sessions to real resources, and updates the contributed session metadata.