Fix hidden Codex subagents and stuck parent sessions (#2068)

* fix(server): prevent hidden Codex subagents and stuck turns

Codex can announce native children only through its mirrored lifecycle stream, while rejected interrupts previously looked like successful local cancellation. Preserve those child announcements and keep manager state active until the provider acknowledges cancellation.

* fix(server): block actions after rejected cancellation

A provider can accept a turn before publishing the turn ID needed to interrupt it. Keep that interval non-cancelable, and prevent reload, replacement, or rewind from proceeding without an acknowledged cancellation.

* fix(server): surface rejected agent cancellations

Replacement prompts and Stop requests could appear accepted after the provider kept ownership of the active turn. Complete replacement cancellation before detaching the stream, and return cancellation failures through the client response.

* fix(server): handle turn completion during cancellation

* fix: finish cancellation lifecycle handling

* fix(server): close remaining cancellation races

* fix(server): settle autonomous cancellations

* fix: honor cancellation failures at call sites

* refactor(server): centralize agent run state

* fix(server): keep pending runs isolated from stale events

* fix: surface remaining cancellation failures

* fix(server): narrow cancellation error handling

packages/protocol/src/messages.ts

@@ -3139,6 +3139,7 @@ export const CancelAgentResponseMessageSchema = z.object({
     requestId: z.string(),
     agentId: z.string(),
     agent: AgentSnapshotPayloadSchema.nullable(),
+    error: z.string().nullable().optional(),
   }),
 });