Support multi-agent agent host session types

[roblourens]

This draft PR updates local and remote agent host sessions to support agent-specific session types and model routing.

Summary:

• Derive remote agent host session types from advertised root-state agents instead of hard-coding Copilot.
• Add local agent host support for multiple root-state agents, including stable existing-session routing and pre-hydration fallback behavior.
• Align session type labels to show the agent name with the host in square brackets, such as Copilot [Local] and Copilot [My Host].
• Fix agent-host language model registration ordering so initial model updates are observed.
• Improve IPC logging so cached root state and later root-state changes are visible in the output channel.

Validation:

• Focused agent-host/session provider tests passed.
• Core typecheck passed with 0 errors.

(Written by Copilot)

[github-actions]

Screenshot Changes

Base: 48409a5d Current: f2cdf295

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[Copilot]

Pull request overview

This PR updates the agent-host-backed sessions (local + remote) to support multiple agents per host by deriving session types dynamically from each host’s advertised rootState.agents, and by aligning session-type IDs/labels so chat routing + model routing stay consistent across boundaries.

Changes:

• Derive remote agent-host session types from rootState.agents (no longer hard-coded to Copilot), and introduce a shared helper to pin the remote session-type wire format.
• Add local agent-host support for multiple advertised agents, including contribution-based fallback before root state hydration and stable routing for existing sessions.
• Wire session-type updates through the sessions layer (ISessionsProvider.onDidChangeSessionTypes → management service → session type picker), plus fix language model registration ordering so initial model updates are observed.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/loggingAgentConnection.ts	Improves IPC logging to include cached root state and root-state change events.
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostChatContribution.ts	Fixes model provider registration ordering so initial model updates are not missed.
src/vs/sessions/SESSIONS_PROVIDER.md	Updates documentation to reflect per-host providers and dynamic session types derived from rootState.agents.
src/vs/sessions/services/sessions/common/sessionsProvider.ts	Adds optional onDidChangeSessionTypes for providers with dynamic session types.
src/vs/sessions/services/sessions/browser/sessionsManagementService.ts	Subscribes to provider session-type changes and updates the global session-type list accordingly.
src/vs/sessions/contrib/remoteAgentHost/test/common/remoteAgentHostSessionType.test.ts	Adds tests pinning the remote session-type ID wire format.
src/vs/sessions/contrib/remoteAgentHost/test/browser/remoteAgentHostSessionsProvider.test.ts	Expands coverage for dynamic agent discovery, session typing, and pending-session type switching.
src/vs/sessions/contrib/remoteAgentHost/common/remoteAgentHostSessionType.ts	Introduces remoteAgentHostSessionTypeId(...) helper to unify routing across boundaries.
src/vs/sessions/contrib/remoteAgentHost/browser/remoteAgentHostSessionsProvider.ts	Implements dynamic session types from root state and supports changing type for pending sessions.
src/vs/sessions/contrib/remoteAgentHost/browser/remoteAgentHost.contribution.ts	Registers per-agent contributions/providers for any advertised agent (not just Copilot).
src/vs/sessions/contrib/localAgentHost/test/browser/localAgentHostSessionsProvider.test.ts	Adds coverage for local multi-agent typing, hydration fallback, and pending-session type switching.
src/vs/sessions/contrib/localAgentHost/browser/localAgentHostSessionsProvider.ts	Implements local multi-agent session typing with contribution fallback + stable handling for existing sessions.
src/vs/sessions/contrib/chat/browser/sessionTypePicker.ts	Refreshes the picker when providers change session types at runtime; merges “supported types” with provider list.
src/vs/platform/agentHost/node/copilot/copilotAgent.ts	Aligns Copilot agent display name with the new session-type labeling conventions.

Copilot's findings

Comments suppressed due to low confidence (1)

src/vs/sessions/contrib/remoteAgentHost/browser/remoteAgentHostSessionsProvider.ts:547

• When rebuilding the pending new session in setSessionType, _selectedModelId is not cleared. Because the rebuilt session has a new id/resource, any previously selected model will carry over to the new agent type and may be invalid for that agent. Clear/reset the pending session’s selected model when the session type changes.

		const rebuilt = this._buildNewSessionData(workspace, newType);
		this._currentNewSession = rebuilt.data;
		this._currentNewSessionStatus = rebuilt.status;

		const fromSession = this._chatToSession(prev);
		const toSession = this._chatToSession(rebuilt.data);
		this._onDidReplaceSession.fire({ from: fromSession, to: toSession });
		return toSession;

• Files reviewed: 14/14 changed files
• Comments generated: 3