v1.1.0-beta.1: add proxy integration tests (62 tests, all green)

Nine end-to-end tests for the proxy feature: 6 network tests exercising
every proxy code path through a real hyper forward proxy (TestProxy) and
a mockito origin, plus 3 CLI tests verifying flag parsing and error handling.

TestProxy binds to 127.0.0.1:0, forwards in absolute-form, counts requests
via an atomic so we can assert traffic actually traversed the proxy.

Key issues resolved during implementation:

- ENV_MUTEX serializes all tests that mutate HTTPS_PROXY/HTTP_PROXY in both
  the unit test module and the integration suite. Without it, parallel test
  execution within a single binary produces nondeterministic failures.

- reqwest's blocking::Client owns an internal tokio Runtime. Dropping it
  inside a #[tokio::test] async fn panics on tokio >= 1.49. All reqwest
  work runs inside spawn_blocking so the Client drops on a thread-pool
  thread where that's permitted.

- service_fn's closure can't carry a return-type annotation, and async
  blocks don't support one either. The handler is extracted to a named
  async fn (proxy_handler) so the compiler can see the concrete
  Result<Response, Infallible> and satisfy serve_connection's Error bound.

Cargo.toml

@@ -63,6 +63,17 @@ assert_cmd = "2.0"
 predicates = "3.1"
 insta = "1.41"
 
+# Proxy integration tests: mockito is the mock origin server; hyper + friends
+# are the forward proxy.  All three are already in Cargo.lock as transitive
+# deps of reqwest — we're just promoting them to explicit dev-deps with the
+# features we actually need.
+mockito = "1.7"
+hyper = { version = "1", features = ["server", "http1", "client"] }
+hyper-util = { version = "0.1", features = ["tokio", "client-legacy", "http1"] }
+http-body-util = "0.1"
+bytes = "1"
+tokio = { version = "1", features = ["full"] }
+
 [build-dependencies]
 ureq = { version = "2.10", features = ["json"] }
 zip = "2.2"