fix(http1): synchronize dispatch shutdown with sends

[fzlzjerry]

Fixes #4122.

Summary

• serialize HTTP/1 envelope publication with receiver shutdown
• close the receiver under the same synchronization boundary before teardown
• add regression coverage for a concurrent send and connection shutdown

Background

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

The shared lock makes publication and HTTP/1 receiver shutdown linearizable.
The HTTP/2 UnboundedSender path remains unchanged.

Validation

• cargo test --features full
• cargo clippy --features full -- -D warnings
• 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
• 500,000-iteration issue stress test: 23 missed drops on master, 0 with this
  change
• giver_queue_throughput benchmark: master 479.70/486.63 ns per iteration;
  this change 471.91/470.48 ns per iteration (within benchmark variance)

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