Add Workers Cache (ctx.cache.purge) to Miniflare

[timhanlon]

I'm currently working on an implementation of Workers Cache. As far as I can tell there’s no local dev story, and the only way to test purge behaviour is deploying to a preview environment on Cloudflare and running E2E tests against it.

The gap turns out to be small. workerd already ships the JS surface scaffolding (api::CacheContext, the purge structs, and the Worker::Api::getCtxCacheProperty() hook), but standalone workerd returns kj::none from the hook, so ctx.cache is undefined under Miniflare. Miniflare's cache simulator already implements the HTTP cache protocol for match/put/delete — it just has no purge-by-tag route.

I got my agents to build a working proof-of-concept across both repos:

• workerd: a StandaloneCacheContext implementing the existing getCtxCacheProperty() hook, plus a CacheClient::purge() that forwards {tags, purgeEverything} to the configured cache service as PURGE /purge-tags (repeated Cache-Tag headers) or DELETE /purge-all. Non-2xx responses map to CachePurgeResult.errors; pathPrefixes is rejected with a clear error (the simulator doesn't index by path). Covered by server tests asserting the exact wire format.
• workers-sdk: the cache worker gains a PURGE /purge-tags handler that scans stored entries' Cache-Tag metadata, and reuses the existing DELETE /purge-all.

Diffs against upstream main: workerd, workers-sdk

I'm happy to open PRs for both if that would be helpful, though I'm not a regular contributor, and this is my first exposure to C++ in about 20 years.