test: cover cut-off request bodies and http/2 clients

[jamiehdev]

This adds integration tests for two untested paths: a request body that fails partway through, and HTTP/2 clients on the TLS listener. The proxy code does not change.

Changes

• tests/integration/body_failure.rs puts a raw TCP origin behind a real proxy socket. The client stops a Content-Length body or a chunked body partway, or sends a malformed chunk size. The tests assert that the origin never gets a whole request. For a half-close, the client gets a 502. For bad chunk framing, actix closes the connection with no response. After each case, /health returns 200.
• tests/integration/http2.rs uses a hyper client that offers only h2 through ALPN and checks Version::HTTP_2 on every response. It covers a proxied GET, a 200 KB POST body, a cache MISS then HIT, and X-Forwarded-Proto: https with X-Forwarded-Host taken from :authority.
• A further HTTP/2 test sends origin responses that hold Connection, Keep-Alive, TE, Upgrade and a field that Connection names. The test requests the response fresh and from the cache. The h2 crate refuses to send TE: gzip, and actix removes only some of these fields itself.

Verification

• A change to pump_request_body that drops the payload error makes the chunked half-close test fail, because the origin then gets a 0\r\n\r\n last chunk.
• The Content-Length test still passes under that change, because hyper also aborts a body that ends before its declared length. The malformed-chunk test also still passes, because actix drops the handler, which cancels the origin request.
• Removing te from the proxy hop-by-hop list resets the h2 stream. Ignoring the URI authority in ClientInfo fails the X-Forwarded-Host assertion.
• Five full local runs and twenty runs of the new tests passed.

[jamiehdev]

@codex review, and also report simplification opportunities in the changed code as P1: logic that duplicates an existing helper, dead code, needless indirection or abstraction, and conditionals or loops with a shorter equivalent. Flag one only when the shorter version behaves the same, and show the replacement code. Do not flag style, naming or formatting.