refactor: fix reactive anti-pattern and cache key in CachedSitemapGetter

[ruibaby]

Summary

Three issues in the previous CachedSitemapGetter implementation:

1. Reactive anti-pattern: Guava Cache.get(key, Callable) was used with .block() inside the callable. This is a reactive anti-pattern — even on Schedulers.boundedElastic(), nesting .block() inside a Guava cache loader risks deadlock under concurrent load.
2. DNS-triggering cache key: SitemapGeneratorOptions (which contains java.net.URL) was used as the cache key. URL.equals() and URL.hashCode() perform DNS resolution on every cache lookup, which is a well-known Java gotcha.
3. Redundant cache.put(): Guava's cache.get(key, callable) automatically stores the callable result — the explicit cache.put() call afterward was redundant.

Changes

• Replace Guava Cache with Caffeine AsyncCache<String, String>
• Use siteUrl.toString() as the cache key (plain String, no DNS lookup)
• The loader returns a CompletableFuture directly from the reactive pipeline (.toFuture()), wrapped in Mono.fromFuture() — no .block() anywhere
• Caffeine automatically deduplicates concurrent loads for the same key

Test plan

• CachedSitemapGetterTest.get() — lister is called exactly once even with 10 concurrent requests hitting the same key
• Verify no blocking calls appear on the reactive event loop thread

Made with Cursor

None

[f2c-ci-robot]

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