Add created property for new chat sessions API

[mjbvz]

The chat sessions API timing property changes are a compile time break but should be runtime backwards compatible. This makes it so that we populate the new created field

[Copilot]

Pull request overview

This PR updates the chat sessions API to add a new required created property for timing information while deprecating the old startTime property. The changes are designed to be runtime backwards compatible by continuing to populate both the new created field and the deprecated startTime field.

Changes:

• Updated the VS Code API definition to add created, lastRequestStarted, and lastRequestEnded timing properties, while deprecating startTime and endTime
• Updated all three chat session providers (Copilot Cloud, Claude, and Copilot CLI) to populate the new created field
• Modified the type definition in copilotcliSessionService.ts to use ChatSessionItem['timing'] for better type safety

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
src/extension/vscode.proposed.chatSessionsProvider.d.ts	Added new timing properties (created, lastRequestStarted, lastRequestEnded) to ChatSessionItem, deprecated old properties (startTime, endTime), and moved API function declarations
src/extension/chatSessions/vscode-node/copilotCloudSessionsProvider.ts	Updated to populate the new created field in addition to the deprecated startTime field
src/extension/chatSessions/vscode-node/claudeChatSessionItemProvider.ts	Updated to populate the new created field in addition to the deprecated startTime field
src/extension/agents/copilotcli/node/copilotcliSessionService.ts	Updated timing type to use ChatSessionItem['timing'] and populated the new created field in all timing object creations