agentHost: adopt notify/sessionSummaryChanged protocol notification

[connor4312]

• Syncs agent-host-protocol to pick up the new
  notify/sessionSummaryChanged notification type, which lets clients
  keep cached session lists in sync without subscribing to every
  session URI individually.
• Emits the notification from AgentHostStateManager: after the session
  reducer runs, an identity check on the summary object detects
  changes, which are debounced (100ms) and flushed as partial diffs.
• Replaces the onDidAction-based subscription-peeking pattern in
  AgentHostSessionListController with the cleaner notification-based
  approach, removing the need for StateComponents/isSessionAction
  imports and the getSubscriptionUnmanaged call.
• Adds unit tests for emission, coalescing, no-op suppression, and
  cleanup on session deletion.

Adopts https://github.com/microsoft/agent-host-protocol/pull/51
Fixes #305330

(Commit message generated by Copilot)

[Copilot]

Pull request overview

This PR updates VS Code’s agent host integration to use the new notify/sessionSummaryChanged protocol notification so clients can keep cached session lists in sync without subscribing to every session URI.

Changes:

• Adds and documents the notify/sessionSummaryChanged protocol notification and updates the protocol sync marker.
• Emits debounced/coalesced sessionSummaryChanged notifications from AgentHostStateManager when session summaries change.
• Switches AgentHostSessionListController to update from notifications instead of peeking subscriptions/actions, and adds unit tests for notification behavior.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionListController.ts	Consume sessionSummaryChanged notifications and maintain a cached summary map for partial updates.
src/vs/platform/agentHost/node/agentHostStateManager.ts	Track last-notified summaries and emit debounced/coalesced sessionSummaryChanged notifications with partial diffs.
src/vs/platform/agentHost/test/node/agentHostStateManager.test.ts	Add tests for emission, debouncing/coalescing, no-op suppression, and deletion cleanup.
src/vs/platform/agentHost/common/state/protocol/version/registry.ts	Register the new notification type in the protocol version registry.
src/vs/platform/agentHost/common/state/protocol/notifications.ts	Define and document the new ISessionSummaryChangedNotification notification type.
src/vs/platform/agentHost/common/state/protocol/.ahp-version	Bump synced agent-host-protocol revision marker.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 2

[Copilot]

ISessionSummaryChangedNotification.changes is typed as Partial<ISessionSummary>, but the doc states identity fields (resource, provider, createdAt) MUST be omitted. Consider tightening the type to Partial<Omit<ISessionSummary, 'resource' | 'provider' | 'createdAt'>> (or a dedicated ISessionSummaryChanges type) so callers can’t accidentally include immutable fields.

Suggested change

	changes: Partial<ISessionSummary>;
	changes: Partial<Omit<ISessionSummary, 'resource' | 'provider' | 'createdAt'>>;

[Copilot]

When applying notify/sessionSummaryChanged, spreading ...n.changes can overwrite immutable summary fields (e.g. resource, provider, createdAt) if a server bug/misbehaving peer sends them, which can desync _cachedSummaries from _items. Since the protocol explicitly treats those fields as immutable, consider explicitly applying only the allowed mutable keys (or filtering them out) instead of a raw spread merge.

Suggested change

	const updated = { ...cached, ...n.changes };
	const mutableChanges = Object.fromEntries(
	Object.entries(n.changes).filter(([key]) => key !== 'resource' && key !== 'provider' && key !== 'createdAt')
	) as Partial<ISessionSummary>;
	const updated = { ...cached, ...mutableChanges };

[awakeairplane]

Disable agent host