[alshdavid]

Summary

Replaces the two reachable panics called out in #25 with graceful handling.

• handle_error now falls back to a bare 500 response if the builder rejects the response, instead of todo!().
• The duplex reader used by body_stream no longer panics with .map_err(|_| panic!()). Read errors — including the writer being dropped mid-stream — now end the body stream cleanly so the client connection closes rather than the task unwinding.

Fixes #25

Notes

• cargo check, cargo clippy --all-targets --workspace -- -D warnings, cargo test (32 passed) and cargo xfmt --check pass locally.
• No new tests: the stream-error path needs a mid-response writer failure to reach; existing behavior is unchanged on the happy path.