Cheroot responds to incomplete requests when the connection is half-closed

[kenballus]

Description of the bug

From RFC 9112:

If a valid Content-Length header field is present without Transfer-Encoding, its decimal value defines the expected message body length in octets. If the sender closes the connection or the recipient times out before the indicated number of octets are received, the recipient MUST consider the message to be incomplete and close the connection.

Cheroot does not enforce this rule. When it receives a request, and the sender half-closes the connection, Cheroot responds regardless of whether the request's body has been fully received.

To reproduce

1. Start a Cheroot-based HTTP server that echos the message body. (e.g., something like this)
2. Send it a request with an incomplete message body, followed by half-closing the socket, and observe that it still responds:

printf 'GET / HTTP/1.1\r\nHost: a\r\nContent-Length: 10\r\n\r\nA' | nc localhost 80

HTTP/1.1 200 OK
Content-type: application/json
Content-Length: 133
Date: Tue, 23 Jul 2024 01:28:24 GMT
Server: Cheroot/10.0.2.dev71+g1ff20b18

{"headers":[["SE9TVA==","YQ=="],["Q09OVEVOVF9MRU5HVEg=","MTA="]],"body":"QQ==","version":"SFRUUC8xLjE=","uri":"Lw==","method":"R0VU"}

5. Decode the response to see that it accepted the incomplete message body:

printf '{"headers":[["SE9TVA==","YQ=="],["Q09OVEVOVF9MRU5HVEg=","MTA="]],"body":"QQ==","version":"SFRUUC8xLjE=","uri":"Lw==","method":"R0VU"}' \
    | jq '.["body"]' \
    | xargs echo \
    | base64 -d \
    | xxd

00000000: 41                                       A