chat (sessions): fix forking AHP chats so the new session opens with a "Forked: " title

[connor4312]

Fixes two user-facing bugs when forking an agent-host (AHP) chat from the Agents
window, and one underlying turn-id mismatch that made forks target the wrong
SDK event boundary on restored sessions:

• agentService: stamp "Forked: " onto the forked session's
  SessionSummary at create time so the sidebar (which renders from
  SessionSummary.title via sessionAdded / sessionSummaryChanged) shows the
  prefix instead of the auto-generated summary inherited from the source.
  Idempotent so re-forking an already-forked session does not double-prefix.
• agentHostSessionHandler: read the forked title back from the freshly
  hydrated session state for the returned IChatSessionItem and drop the
  client-side SessionTitleChanged dispatch.
• chatForkActions: route the contributed-session fork path through
  ForkConversationAction._openForkedSession so the Agents-window override
  applies; inlines the helper to avoid a callback hop.
• localChatSessions.contribution: in the Agents-window override, wait for
  the forked resource to appear and then open it via
  sessionsManagementService.openSession for every session type (not just
  the local VS Code chat type) -- the previous fallback called
  chatWidgetService.openSession, which is a no-op in the Agents window
  because there is no ChatViewPane.
• mapSessionEvents: when restoring a session from disk, seed the
  protocol turn id from the SDK envelope id (the same value
  setTurnEventId persists to turns.event_id). This makes the restored
  state.turns[].id round-trip back to the SDK boundary id that the
  sessions.fork / history.truncate RPCs operate on.
• sessionDatabase: getNextTurnEventId now resolves the source row by
  either turns.id (live request_xxx) OR turns.event_id (SDK envelope id),
  so fork works for both freshly-dispatched and restored turn ids.

Fixes #317839

(Commit message generated by Copilot)

[Copilot]

Pull request overview

This PR fixes AHP chat forking in the Agents window so forked sessions open correctly, carry a visible "Forked: " title, and use restored turn IDs that align with SDK event boundaries.

Changes:

• Moves forked-session title stamping into Agent Host session creation and reads it back when returning forked chat items.
• Routes contributed chat forks through the overridable open path so the Agents window can open forked sessions via ISessionsManagementService.
• Aligns restored turn IDs with SDK envelope event IDs and adds database/test coverage for next-turn event lookup by either turn ID or event ID.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionHandler.ts	Uses hydrated forked session summary title for the returned forked chat item label.
src/vs/workbench/contrib/chat/browser/actions/chatForkActions.ts	Routes contributed-session forks through _openForkedSession so subclass overrides apply.
src/vs/sessions/contrib/providers/localChatSessions/browser/localChatSessions.contribution.ts	Opens forked sessions in the Agents window through session management for local and AHP sessions.
src/vs/platform/agentHost/test/node/sessionDatabase.test.ts	Adds tests for resolving next turn event IDs by both stored turn IDs and SDK event IDs.
src/vs/platform/agentHost/test/node/copilotAgentSession.test.ts	Updates and adds replay tests for SDK envelope IDs as restored turn IDs.
src/vs/platform/agentHost/node/sessionDatabase.ts	Allows getNextTurnEventId to resolve source turns by turns.id or turns.event_id.
src/vs/platform/agentHost/node/copilot/mapSessionEvents.ts	Seeds restored turn IDs from SDK envelope event IDs when available.
src/vs/platform/agentHost/node/agentService.ts	Prefixes forked AHP session summaries with "Forked: " at creation time, idempotently.

Copilot's findings

• Files reviewed: 8/8 changed files
• Comments generated: 0