fix(native-chat): never let the pre-stop snapshot hold a chat's stop

Eviction now drains delivered events before quit's resume-offer snapshot. An
unbounded wait there sits ahead of the provider stop, so a sink whose journal
write stalls kept the child running until the step deadline aborted the
eviction. The offer is advisory: bound the drain and stop the child regardless.

Co-Authored-By: Claude <noreply@anthropic.com>

src/main/native-chat/agent-session-wire/structured-agent-session-eviction.test.ts

@@ -94,6 +94,31 @@ describe('structured agent session eviction', () => {
     ])
   })
 
+  it('still stops the child when the pre-stop snapshot cannot drain the sink', async () => {
+    vi.useFakeTimers({ toFake: ['setTimeout', 'clearTimeout'] })
+    try {
+      const ctx = context()
+      const snapshot = vi.fn()
+      ctx.beforeProviderChildStop = snapshot
+      // A journal write that never settles, then a healthy sink once the child is stopped.
+      ctx.eventSink.drained = vi
+        .fn()
+        .mockReturnValueOnce(new Promise(() => {}))
+        .mockResolvedValue({
+          ok: true
+        }) as unknown as StructuredAgentSessionEvictionContext['eventSink']['drained']
+
+      const eviction = evictStructuredAgentSession(ctx)
+      await vi.advanceTimersByTimeAsync(1_000)
+
+      expect(snapshot).toHaveBeenCalledOnce()
+      expect(ctx.adapter.closeSession).toHaveBeenCalledOnce()
+      await eviction
+    } finally {
+      vi.useRealTimers()
+    }
+  })
+
   it('aborts after a failed drain barrier without unbinding or forgetting the session', async () => {
     const ctx = context()
     ctx.eventSink.drained = vi.fn(async () => {

src/main/native-chat/agent-session-wire/structured-agent-session-eviction.ts

@@ -23,6 +23,7 @@ import {
   type StructuredAgentSessionAdapter
 } from './structured-agent-session-adapter'
 import type { DeferredStructuredAgentSessionEventSink } from './structured-agent-session-event-sink'
+import { withTimeout } from '../../../shared/promise-timeout-fallback'
 
 export type StructuredAgentSessionEvictionContext = {
   sessionId: string
@@ -50,6 +51,9 @@ export type StructuredAgentSessionEvictionContext = {
   releaseLease: () => Promise<void>
 }
 
+/** The resume offer is advisory; a stalled sink must not hold the child's stop behind it. */
+const SNAPSHOT_DRAIN_TIMEOUT_MS = 1_000
+
 export type StructuredAgentSessionEvictionStep = {
   name: string
   run: (context: StructuredAgentSessionEvictionContext) => Promise<void> | void
@@ -65,7 +69,7 @@ export const STRUCTURED_AGENT_SESSION_EVICTION_STEPS: readonly StructuredAgentSe
           return
         }
         // Events the provider already delivered are part of what the sidebar showed at the stop.
-        await context.eventSink.drained()
+        await withTimeout<unknown>(context.eventSink.drained(), SNAPSHOT_DRAIN_TIMEOUT_MS, null)
         try {
           context.beforeProviderChildStop()
         } catch {