Warning-free build, cacheability refactor, and real-world sample (API + Redis + Polly)

[FranRuiz98]

What

Three independent pieces of housekeeping and developer experience:

1. Warning-free build (25b1621): resolves every xUnit analyzer warning in the test project (xUnit1051: use TestContext.Current.CancellationToken). The full solution now builds with 0 warnings, matching the repo's contribution rule.
2. Refactor: converts the cacheability status-code switch statement in CachingMiddleware to a switch expression. No behavior change — same status codes, same explicit-freshness guard for 404/405/410/414.
3. Real-world sample (samples/): a runnable deployment proving the full recommended pipeline against real infrastructure — Kestrel origin API + Redis-backed DistributedCacheStore + Polly (retry with exponential backoff + timeout) chained below the coalescer, orchestrated with docker-compose running two client replicas.

Why

• The analyzer warnings were the last noise in an otherwise zero-warning build.
• The demo project uses a fake in-process handler by design; the sample fills the gap it can't: real network, real ASP.NET-generated headers, real Redis serialization, and the multi-instance story (cache shared via Redis, coalescing per-process — documented explicitly in samples/README.md).

Reviewer notes

• Samples are intentionally NOT in Stampede.Http.slnx, so CI and the normal dev loop stay Docker-free. They reference the library by ProjectReference, so they always exercise current source.
• Validated end-to-end locally (real Kestrel + Redis container + two client instances): coalescing collapsed a 10-caller burst to 1 origin call (coalescing.deduplicated = 9), sub-millisecond Redis hits with Age, 304 conditional revalidation on expiry, stale-while-revalidate background refresh, Polly retries visible on 503 bursts, and a POST /catalog invalidation observed by both client instances through the shared store.
• The docker compose up --build image builds themselves were not exercised (only local dotnet run + a Redis container); a root .dockerignore is included to keep bin/obj out of the build context.
• Found during sample validation, out of scope here: after a 304 revalidation the Age header keeps growing instead of resetting (RFC 9111 §4.3.4 / §4.2.3). Tracked as a follow-up task.

Test plan

• dotnet build Stampede.Http.slnx -c Release → 0 warnings, 0 errors
• dotnet test Stampede.Http.Tests -c Release → 343/343 passing
• Manual e2e of the sample as described above

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