perf(session): rebuild session payload from latest store state in debounce

Replace the closed-over `state` snapshot captured at timer-schedule time
with `store.getState()` inside the setTimeout callback. Today this is
behaviorally equivalent because `buildWorkspaceSessionPayload` reads only
SESSION_RELEVANT_FIELDS (the same fields gating the timer reset), but a
future refactor that adds a non-relevant field read to the payload builder
would silently start emitting stale values without this guard.

Also tighten the cleanup test: mutate the store after `cleanup()` and assert
no persist, so a regression where the timer is cancelled but the listener
is left subscribed would now fail rather than pass.

Co-authored-by: Orca <help@stably.ai>

src/renderer/src/lib/session-write-subscriber.test.ts

@@ -127,5 +127,13 @@ describe('createSessionWriteSubscriber', () => {
     vi.advanceTimersByTime(200)
 
     expect(persist).not.toHaveBeenCalled()
+
+    // Why: without this second mutation, the assertion above only proves the
+    // pending timer was cancelled — a regression where cleanup() forgot to
+    // unsub() would still pass. Mutating after cleanup verifies the listener
+    // was detached and no new timer is queued.
+    useAppStore.setState({ activeTabId: 'tab-2' })
+    vi.advanceTimersByTime(200)
+    expect(persist).not.toHaveBeenCalled()
   })
 })

src/renderer/src/lib/session-write-subscriber.ts

@@ -60,7 +60,15 @@ export function createSessionWriteSubscriber({
     }
     timer = setTimeout(() => {
       timer = null
-      persist(buildWorkspaceSessionPayload(state))
+      // Why: rebuild from the freshest store state rather than the snapshot
+      // captured when this timer was scheduled. Today this is equivalent
+      // because buildWorkspaceSessionPayload reads only SESSION_RELEVANT_FIELDS
+      // (the same fields gating the timer reset), so the captured `state` is
+      // already current for those fields. Calling getState() guards against a
+      // future refactor that adds a non-relevant field read to the payload
+      // builder — without this, such a change would silently start emitting
+      // stale values for that field.
+      persist(buildWorkspaceSessionPayload(store.getState()))
     }, debounceMs)
   })