Fix streaming-body watchdog bug (review #009 C2) + dead-upstream test wiring (C3)

C2: IdleTrackingService decremented in_flight when the handler returned
the Response, but for streaming responses (e.g. git clone) the body is
still in flight, so the watchdog could kill the connection mid-stream.
Fix: wrap the response body in IdleTrackingBody which owns the decrement,
releasing it on body EOF/error/drop (guarded against double-decrement by
an AtomicBool) and calling touch() on each data frame. The handler-level
decrement now only happens on the Err branch. Verified by a reproducer
test (streaming_body_not_killed_by_idle_watchdog) that streams 15 chunks
over 3s with a 500ms idle timeout — red on 4ab8c51, green with this fix.

C3: the two pre-existing idle-timeout tests pointed at 127.0.0.1:18080
where nothing listens, so they validated watchdog behaviour against an
immediate 504 error response (HTTP/1.1 prefix + in_flight==1 hold for
a 504 just as for a 200) and never exercised a real upstream round-trip.
Rewired them to spawn a real TestUpstream and assert HTTP/1.1 200 OK.
Removed the dead no-arg make_proxy_router/start_test_https_server wrappers.

Adds http-body as a direct dep (for the Frame/Body trait) and
tokio-stream as a dev-dep (for the slow-stream test helper).

cargo test: 226 unit + 40 integration green; cargo clippy --all-targets clean.

C1 (deploy to dev1) remains open and out of scope for this repo session.

Cargo.toml

@@ -17,6 +17,7 @@ axum = "=0.8.9"
 tokio = { version = "=1.45.1", features = ["full"] }
 hyper = "=1.6.0"
 hyper-util = { version = "=0.1.17", features = ["client-legacy", "http1", "http2", "tokio"] }
+http-body = "=1.0.1"
 http-body-util = "=0.1.3"
 hyper-rustls = { version = "=0.27.9", features = ["http1", "http2"] }
 rustls-native-certs = "=0.8.1"
@@ -47,4 +48,5 @@ hex = "0.4"
 [dev-dependencies]
 rcgen = "=0.13"
 reqwest = { version = "=0.12", features = ["json"] }
-tempfile = "=3.20"
+tempfile = "=3.20"
+tokio-stream = "=0.1.17"