Closes #4176

Motivation

When an HTTP/1 client receives a complete response while its request body is
still being written, the connection can remain parked in
(Reading::KeepAlive, Writing::Body). The response has completed, but the
connection is neither reusable nor closed, so pooled clients can leak sockets.

Solution

Close this terminal client state in State::try_keep_alive. The transition is
guarded with T::is_client() because the same state shape is valid while a
server is still streaming its response. Regression tests cover both roles:
clients close and servers retain the streaming body.

The close-vs-drain choice follows the issue's terminal-outcome contract: a
client that has already received a complete response cannot safely reuse the
connection while its request body is unfinished. I am happy to adjust this to
a drain policy if maintainers prefer that semantics.

Tests

• cargo test --features full --lib (117 passed, 6 ignored)
• deterministic client reproducer: unchanged master fails; this commit
  closes the connection
• server streaming harness: delayed response chunks remain intact
• rustfmt --check --edition 2021 src/proto/h1/conn.rs
• git diff --check

AI assistance was used for code navigation and test drafting. The patch,
reproduction, and verification were reviewed by the author and an independent
non-author agent. Full cross-platform CI remains to be run by GitHub.