Add a smoke test for real servers, and show why transport errors happen

The suite runs against mock servers written from the upstream source, so
it cannot catch a place where a real server disagrees with that reading.
`examples/smoke_test.rs` can: it lists every resource, checks the
pagination walk against a single oversized request, probes the per_page
cap, and exercises statics, search and health_check. The upload wire
format (POST plus `_method=PUT`) was read from the framework source
rather than observed, so its write tier is the real verification of it.

It prints PASS/FAIL per check and never the contents of a record.

Writes are guarded. `IN_SMOKE_WRITE_HOST` must equal the host in
`IN_BASE_URL`, so a write can only reach a server that was named
explicitly. `IN_BASE_URL` has no default, unlike the other example,
because defaulting to the hosted app is the wrong failure for a tool
that can write. The write tier leaves a gap in the client number
sequence and a soft-deleted row even after cleanup, so it is documented
as disposable-instance only.

Running it against a dead port exposed a diagnosability gap:
`Error::Http` printed "error sending request for url (...)" and nothing
else. reqwest's Display stops at the outermost layer, and the reason
("Connection refused", a DNS failure, a TLS error) lives in `source()`.
The error now renders its whole cause chain.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -68,6 +68,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   from a proxy or a login redirect from a wrong base URL.
 - API errors with an empty or message-less body now report the status reason
   ("Not Found") instead of rendering blank.
+- `Error::Http` now renders its whole cause chain. `reqwest`'s own message
+  stops at "error sending request for url (...)", leaving out the part that
+  says why — "Connection refused", a DNS failure, a TLS handshake error — so
+  anyone printing the error had no way to tell which it was.
 
 ### Fixed