test(integration): add caching lifecycle tests for sourcedFrom, SWR, and invalidation

Adds integrationTests/server/caching.test.ts covering the full cache
lifecycle end-to-end via a fake HTTP source server (modelled after the
embed-directive test pattern):

- sourcedFrom(customSource): cache miss calls source; hit serves from cache
- sourcedFrom(source, {replicationSource: true}): single-node populate-on-miss
- allowStaleWhileRevalidate(): stale value returned immediately; background
  refresh updates cache; next request gets fresh value
- Composite cache key (deviceType prefix in ID): cross-device isolation
- HTTP DELETE invalidation: re-fetch from origin after cache entry deleted

Closes #1189

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: github-actions[bot]