Mark many chat session related types as readonly

[mjbvz]

These types are generally expected to be immutable. This makes them easier to reason about too

[Copilot]

Pull request overview

Marks chat session-related types as readonly to better reflect their intended immutability, and updates affected call sites to avoid mutating session DTOs/items in-place.

Changes:

• Add readonly modifiers to multiple chat session interfaces (items, options, timing, stats).
• Update MainThreadChatSessions to avoid mutating Dto<IChatSessionItem> and to compute/return changes without assigning back to the DTO.
• Update sessions web test mock to avoid mutating IChatSessionItem.timing in-place.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/vs/workbench/contrib/chat/common/chatSessionsService.ts	Makes various chat session-related interfaces readonly (items/options/file change metadata).
src/vs/workbench/contrib/chat/common/chatService/chatService.ts	Makes session timing and stats types readonly.
src/vs/workbench/api/browser/mainThreadChatSessions.ts	Stops mutating incoming session DTOs; uses local variables/copies for overrides and changes.
src/vs/sessions/test/web.test.ts	Adjusts mock session item updates to work with readonly timing/items.