refactor: update chat session item handling to use new request structure

[TylerLeonhardt]

minimizes the API for the newChatSessionItemHandler to only receive the prompt & command.

[Copilot]

Pull request overview

Refactors the “new chat session item” flow so the session-item controller handler only receives the minimal initial request data (prompt + optional command), instead of the full request structure.

Changes:

• Narrow proposed API ChatSessionItemControllerNewItemHandlerContext.request to { prompt, command? }.
• Introduce IChatNewSessionRequest and plumb it through workbench chat session services + IPC ($newChatSessionItem).
• Update call sites/tests to use the new request shape.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/vscode-dts/vscode.proposed.chatSessionsProvider.d.ts	Updates proposed API handler context to accept only prompt/command.
src/vs/workbench/contrib/chat/common/chatSessionsService.ts	Adds IChatNewSessionRequest and updates service/controller method signatures.
src/vs/workbench/contrib/chat/browser/chatSessions/chatSessions.contribution.ts	Updates ChatSessionsService implementation to accept IChatNewSessionRequest.
src/vs/workbench/contrib/chat/test/common/mockChatSessionsService.ts	Adjusts mock service signature to match new request type.
src/vs/workbench/api/common/extHostChatSessions.ts	Updates ext host handler invocation to pass prompt/command only.
src/vs/workbench/api/common/extHost.protocol.ts	Updates IPC shape for $newChatSessionItem to use the new request type.
src/vs/workbench/api/browser/mainThreadChatSessions.ts	Updates main thread controller method signature to accept the new request type.
src/vs/workbench/api/browser/mainThreadChatAgents2.ts	Updates call site to construct { prompt, command? } from the agent request.

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