Do not have pending title show up in session list title

[anthonykim1]

Coming from: #311535 and #312038

Fixing issue where the session list title in the agents app temporarily changes in/out from original set title to per-request related content.
/cc @DonJayamanne @roblourens

Screen.Recording.2026-04-26.at.10.13.28.PM.mov

The session list was temporarily using the in-flight request text leading to undesirable UI.

[Copilot]

Pull request overview

This PR aims to prevent session titles (especially for Copilot CLI-backed sessions) from briefly switching to in-flight/request-derived text, keeping the Agents app session list title stable and consistent across refreshes and renames.

Changes:

• Adjust Copilot CLI session-title resolution to avoid using pending prompts for already-established sessions, and add a regression test.
• Update the Agents app Copilot sessions provider to prefer live chat-model custom titles and to keep locally-renamed titles stable across model refreshes.
• Add provider-level test coverage for “rename then refresh” behavior.

Show a summary per file

File	Description
src/vs/sessions/contrib/copilotChatSessions/test/browser/copilotChatSessionsProvider.test.ts	Expands test harness stubs and adds a regression test ensuring a renamed CLI title survives later session model refreshes.
src/vs/sessions/contrib/copilotChatSessions/browser/copilotChatSessionsProvider.ts	Adds title override plumbing (session/chat), display-title resolution via IChatService.getSession, and throttled cache refresh handling.
extensions/copilot/src/extension/chatSessions/copilotcli/node/test/copilotCliSessionService.spec.ts	Adds a regression test ensuring list labels don’t switch to pending prompt text for established sessions.
extensions/copilot/src/extension/chatSessions/copilotcli/node/copilotcliSessionService.ts	Removes “pending prompt” from established-session title precedence and documents the intended behavior.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 4