fix: retain digest retry client through response body

[ryanfowler]

Summary

• retain the temporary digest retry client until the authenticated response body is consumed
• prevent Hyper from canceling the retry connection when the temporary client pool is dropped
• preserve bounded cleanup for large digest challenge bodies

Validation

• cargo fmt --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked --all-features --lib --bins
• digest drain integration test passed 20 consecutive runs