feat: optional response caching via DI (ITvdbCache) — no-op by default, cache-call when registered

[ChrisonSimtian]

Description

TheTVDB recommends callers cache/proxy responses; today this client is deliberately AS IS with none. Add an opt-in caching layer via DI using our own ITvdbCache abstraction:

• If no ITvdbCache is registered → live calls (current behaviour).
• If one is registered → cache-call (serve from cache, else fetch + store).

Pattern (the clean way)

Null Object + a caching DelegatingHandler — DI-native, composes with the existing auth (and envelope-unwrap) handlers in the HttpClient pipeline:

• ITvdbCache (in TvdbClient.Abstractions) — TryGetAsync(key) / SetAsync(key, value, ttl).
• NoOpTvdbCache — pass-through; registered by AddTvdbClient via TryAddSingleton<ITvdbCache, NoOpTvdbCache>() so a consumer-registered implementation wins.
• CachingHandler : DelegatingHandler — keys on request method+URI; on GET, serves a cached response or forwards + stores. No-op cache ⇒ effectively transparent.

Consumer usage:

services.AddSingleton<ITvdbCache, MyRedisTvdbCache>();  // opt in
services.AddTvdbClient(config);                          // no-op stays if the above is absent

Notes / open questions

• Cache key + TTL policy (per-endpoint? honor any Cache-Control TheTVDB sends?).
• HTTP-level (handler, caches the raw enveloped body — simplest) vs typed-level (caches deserialized DTOs).
• Ship as part of the core, or a separate TvdbClient.Caching package with in-memory + a sample distributed impl.

🤖 Generated with Claude Code