Fix dangling promise in unstable-cache (#79248)

While investigating an issue with intermittent `unstable_cache` calls
failing to set noticed we have these dangling promises which cause
"racey" behavior with setting to a cache in environments where the
function stops after the request is finished.

These have been present since
b305c2d
altho since racey may or may not have been noticed.

x-ref: [slack
thread](https://vercel.slack.com/archives/C02K2HCH5V4/p1747322956802819?thread_ts=1746633220.634939&cid=C02K2HCH5V4)

Author: ijjk

.changeset/loose-cows-pump.md

@@ -0,0 +1,5 @@
+---
+'next': patch
+---
+
+Fix dangling promise in unstable_cache

packages/next/src/server/web/spec-extension/unstable-cache.ts

@@ -275,7 +275,14 @@ export function unstable_cache<T extends Callback>(
         )
 
         if (!workStore.isDraftMode) {
-          cacheNewResult(
+          if (!workStore.pendingRevalidates) {
+            workStore.pendingRevalidates = {}
+          }
+
+          // We need to push the cache result promise to pending
+          // revalidates otherwise it won't be awaited and is just
+          // dangling
+          workStore.pendingRevalidates[invocationKey] = cacheNewResult(
             result,
             incrementalCache,
             cacheKey,
@@ -330,7 +337,11 @@ export function unstable_cache<T extends Callback>(
           cb,
           ...args
         )
-        cacheNewResult(
+
+        // we need to wait setting the new cache result here as
+        // we don't have pending revalidates on workStore to
+        // push to and we can't have a dangling promise
+        await cacheNewResult(
           result,
           incrementalCache,
           cacheKey,