test(ui): stub toast dismissal in the pending-compaction fixture (#1097)

## Problem

Since #1096, the UI unit job on `dev` fails in two
`instances-pending-compaction.test.ts` tests, with `TypeError:
toast.dismiss is not a function` (also in #1096's own PR run).

#1096 shows one bubble per session, so a later compaction end dismisses
the session's earlier bubble. These tests end several compactions in one
session. Under Node with tsx, `import toast from "solid-toast"` yields
the CommonJS module object, which has no `dismiss`. The fixture replaced
only `toast.custom`, so the dismissal threw.

The app is not affected: Vite resolves the real `toast` function, which
has `dismiss`.

## Fix

The fixture also replaces `toast.dismiss` with a no-op and restores it
after each test, as `session-lifecycle-signals.test.ts` already does.
Test-only change.

## Validation

- Reproduced on unmodified `dev` (`333d73452`): 2 of 7 tests in the file
fail.
- With the fix:
- `instances-pending-compaction`, `compaction-notice` and
`session-lifecycle-signals`: 18/18;
  - full UI unit suite: 1694/1694;
  - UI typecheck.

packages/ui/src/stores/instances-pending-compaction.test.ts

@@ -34,11 +34,13 @@ async function until(check: () => boolean) {
 }
 const originalPending = serverApi.getPendingRequests, originalWorktrees = serverApi.fetchWorktrees
 const originalClient = sdkManager.createClient
-const originalToast = toast.custom
+const originalToast = toast.custom, originalDismiss = toast.dismiss
 let sequence = 0, brokerReads = 0
 const fixtures: ReturnType<typeof harness>[] = []
 beforeEach(() => {
   toast.custom = () => "fixture-toast"
+  // A later compaction end replaces the session's earlier bubble.
+  toast.dismiss = () => {}
   brokerReads = 0
   serverApi.getPendingRequests = async (id, directories) => {
     brokerReads++
@@ -51,6 +53,7 @@ beforeEach(() => {
 })
 afterEach(() => {
   toast.custom = originalToast
+  toast.dismiss = originalDismiss
   for (const h of fixtures.splice(0)) {
     removeInstance(h.id, { authoritative: false })
     setSessions((previous) => { const next = new Map(previous); next.delete(h.id); return next })