Fix cancelled subrequest finish losing Done

[pederbe]

Fix cancelled subrequest finish losing Done

Summary

A cancelled legacy subrequest finish can mark the body complete while its terminal Done task is still waiting for channel capacity. Retrying finish then becomes a no-op and loses that task.

Reserve channel capacity before completing the body, then send Done synchronously. Preserve the existing premature-body and closed-channel error behavior. Add deterministic coverage for cancellation and retry, upgraded sessions, repeated and empty completion, and terminal errors.

Fixes #933.

Testing

• Reproduced both content-length and until-close regressions against the original implementation.
• cargo test --locked -p pingora-core --lib protocols::http::subrequest -- --nocapture: 85 passed.
• cargo test --locked -p pingora-core --lib: 591 passed, 2 ignored.
• cargo fmt --all -- --check
• cargo check --locked --workspace
• cargo clippy --locked -p pingora-core --all-targets -- --allow=unknown-lints --deny=warnings