add /models to agents window new input

[justschen]

/models now works in the new agents window input

[Copilot]

Pull request overview

Adds support for /models in the Agents window new chat input by introducing a per-input model picker opener service and wiring existing model picker widgets into it.

Changes:

• Adds INewChatModelPickerService and registers /models as an immediate slash command.
• Scopes model picker openers through NewChatInputWidget child instantiation.
• Updates Copilot/agent-host model pickers, docs, and tests to use the scoped service.

Show a summary per file

File	Description
src/vs/sessions/contrib/chat/browser/newChatModelPicker.ts	Adds the scoped model picker opener service.
src/vs/sessions/contrib/chat/browser/newChatInput.ts	Creates a per-input scoped service and uses it for slash commands/config toolbar.
src/vs/sessions/contrib/chat/browser/slashCommands.ts	Adds the /models slash command.
src/vs/sessions/contrib/chat/test/browser/newChatModelPicker.test.ts	Tests scoped opener isolation.
src/vs/sessions/contrib/providers/copilotChatSessions/browser/copilotChatSessionsActions.ts	Wires local Copilot model picker into the scoped service.
src/vs/sessions/contrib/providers/copilotChatSessions/browser/modelPicker.ts	Wires cloud Copilot model picker into the scoped service.
src/vs/sessions/contrib/providers/copilotChatSessions/test/browser/modelPickerDelegate.test.ts	Stubs the new service for existing picker tests.
src/vs/sessions/contrib/providers/copilotChatSessions/COPILOT_CHAT_SESSIONS_PROVIDER.md	Documents scoped instantiation requirements for model pickers.
src/vs/sessions/contrib/providers/agentHost/browser/agentHostModelPicker.ts	Wires desktop agent-host model picker into the scoped service.
src/vs/sessions/contrib/providers/agentHost/browser/mobile/mobileChatInputConfigPicker.ts	Wires mobile agent-host config picker into the scoped service.

Copilot's findings

• Files reviewed: 10/10 changed files
• Comments generated: 1