Memory exhaustion via late multipart size enforcement

Resource exhaustion in aio-libs aiohttp before 3.13.4 allows remote attackers to force excessive memory allocation via multipart requests. Request.post() in aiohttp/web_request.py previously awaited field.read() for an ordinary multipart field before adding its decoded length to the client_max_size counter, so the entire field was buffered before rejection. The affected application must call Request.post() on attacker-controlled multipart data; the resulting temporary memory pressure can interrupt service without disclosing or altering data.