fix: use process.nextTick instead of setImmediate for background revalidation

setImmediate doesn't work properly with fake timers in tests, causing
background revalidation to not trigger. process.nextTick works correctly
with both real time and fake timers.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Matteo Collina <hello@matteocollina.com>

Author: mcollina

lib/interceptor/cache.js

@@ -254,7 +254,7 @@ function handleResult (
       sendCachedValue(handler, opts, result, age, null, true)
 
       // Start background revalidation (fire-and-forget)
-      setImmediate(() => {
+      process.nextTick(() => {
         let headers = {
           ...opts.headers,
           'if-modified-since': new Date(result.cachedAt).toUTCString()