fix(http1): synchronize dispatch shutdown with sends

[fzlzjerry]

Fixes #4122.

Summary

• close the HTTP/1 dispatch receiver before teardown
• continue polling the closed receiver until a concurrent synchronous send has either published its envelope or observed the closure
• add a regression test that exercises the close/send race 10,000 times

Background

Tokio's unbounded channel reserves message capacity before it publishes the
envelope into the channel list. Receiver shutdown could close the channel and
observe a pending receive during that window. The send would then finish after
shutdown, leaving the request owned by the channel until the remaining
SendRequest was dropped.

After closing the receiver, Pending means that such a pre-close send is still
in flight. Retrying the existing single-poll receive until it yields the
envelope or reports the closed channel removes that window without introducing
an additional synchronization object. The HTTP/2 path remains unchanged.

Validation

• cargo test --features full (309 passed, 10 ignored)
• cargo +1.97.1 clippy --features full -- -D warnings
• cargo fmt --all -- --check
• cargo check --no-default-features --features client,http1
• cargo check --no-default-features --features client,http2
• cargo check --no-default-features --features client,http1,http2
• minimal dependency resolution followed by cargo check --features full

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[fzlzjerry]

Updated the implementation in 333d33a to remove the added Arc<Mutex>. Shutdown now closes the receiver and polls its existing receive future until a synchronous send that already reserved capacity either publishes its envelope or the channel terminates.

The new run passes the race regression, stable/beta/nightly tests, MSRV, minimal versions, Miri, features, docs, and the remaining checks. The sole red Linter job is Rust 1.98 reporting three new lints in untouched files (src/body/incoming.rs, src/client/conn/http1.rs, and src/client/conn/http2.rs); cargo +1.97.1 clippy --features full -- -D warnings passes locally. The PR diff remains limited to the dispatch shutdown path and its regression test.