Adopt copilot/sdk session-title APIs and sync title across session list, editor, CLI

[anthonykim1]

Resolves: #311535

• Renaming a Copilot CLI session title from VS Code now sticks everywhere it
  should — sessions list, chat editor tab/header, and copilot --resume in
  the terminal.

• Adopts the copilot/sdk session-title APIs so the SDK is the source of truth
  instead of a local label cache.

Side fix after some surprise 😮:

Noticed the chat panel/header was not syncing on rename even when
the sessions list was. The two surfaces share no state:

• Sessions list is driven by ChatSessionItem.label from the contributed
  item provider.
• Chat editor tab / panel header is driven by the open chat model's title,
  which only updates when something fires setCustomTitle on the model.

This PR would bridge the gap there as well. -> In mainThreadChatSessions so a label change on a
live session propagates into the chat model.

AFTER PR:

How titles flow:

• Manual rename in VS Code writes to the SDK's name field.
• AI-generated titles (from the chat title provider) write to the SDK's summary field.
• copilot --resume renames write to name too, and VS Code picks them up the next time it reads metadata.
• The sidebar and the chat editor header now go through the same resolver, so they always show the same thing.

How a title is picked, in order:

1. An explicit title if there is one (live session title, SDK name, or legacy custom title).
2. A derived label we cached on a previous pass.
3. The pending prompt, for sessions that just started.
4. A clean summary from metadata (skipped if it looks truncated — ends with ... or contains <).
5. The first user message from session history (cached if we find one).
6. The raw summary as a last resort, just for display — not cached.

A couple of things worth calling out:

• Both renameSession and updateSessionSummary clear the cached derived label, so a brand-new title is never hidden behind an old cached one.
• Before this PR, the sidebar and the editor had slightly different orders of precedence and only the sidebar used the cache, so they could disagree. They share one path now.

/cc @DonJayamanne

[github-actions]

Screenshot Changes

Base: 01ae82da Current: d81e2476

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[Copilot]

Pull request overview

This PR adopts the @github/copilot/sdk session title APIs as the source of truth for Copilot CLI session titles, and ensures title changes propagate across the VS Code sessions list, the chat editor tab/header, and terminal resume flows.

Changes:

• Propagate session-item label changes to the active chat model so editor/header titles stay in sync.
• Switch Copilot CLI session title retrieval/updates to SDK-backed metadata (name/summary) and add SDK summary update support.
• Update Copilot CLI session service logic, mocks, and tests to validate rename/summary/title synchronization.

Show a summary per file

File	Description
src/vs/workbench/api/browser/mainThreadChatSessions.ts	Syncs contributed session item label changes into the live chat model title.
extensions/copilot/src/extension/chatSessions/vscode-node/copilotCLIChatSessionsContribution.ts	After generating a title, updates the SDK session summary so CLI/VS Code surfaces stay consistent.
extensions/copilot/src/extension/chatSessions/vscode-node/copilotCLIChatSessions.ts	Stages generated titles for brand-new sessions and uses SDK getSessionTitle when building list items.
extensions/copilot/src/extension/chatSessions/copilotcli/vscode-node/customSessionTitleServiceImpl.ts	Makes title generation return a value without persisting it, leaving persistence to callers.
extensions/copilot/src/extension/chatSessions/copilotcli/node/test/testHelpers.ts	Extends SDK session mocks with name and title update methods + metadata support.
extensions/copilot/src/extension/chatSessions/copilotcli/node/test/copilotCliSessionService.spec.ts	Adds tests for rename/summary updates and staged-title sync into the SDK.
extensions/copilot/src/extension/chatSessions/copilotcli/node/copilotcliSessionService.ts	Implements SDK-backed title querying + writable-session operations for rename/summary updates and staged-title syncing on session creation.

Copilot's findings

• Files reviewed: 7/7 changed files
• Comments generated: 3

[anthonykim1]

agentsPending.mov

Maybe what's causing the temporary title change? @roblourens @DonJayamanne

This is meant to ensure title is synced across session list, chat editor header though.
Edit: Addressed issue where pending title would flicker in agents app in #312657

[DonJayamanne]

Thanks @anthonykim1 ,