Describe the bug

BodyPartReader.read_chunk(size) picks its read strategy by testing self._length for truthiness:

if self._length:
    fresh = await self._read_chunk_from_length(want)
else:
    fresh = await self._read_chunk_from_stream(want)

self._length is int | None. A body part with an explicit Content-Length: 0 header (legal outside multipart/form-data, e.g. multipart/mixed or multipart/related, where the empty-part special case that nulls out _length does not apply) sets self._length = 0, which is falsy. The reader then falls through to _read_chunk_from_stream, which asserts the requested chunk size is at least the boundary length:

async def _read_chunk_from_stream(self, size: int) -> bytes:
    assert (
        size >= self._boundary_len
    ), "Chunk size must be greater or equal than boundary length + 2"

Any caller asking read_chunk() for a chunk smaller than the boundary length hits that assert instead of getting back an immediate empty chunk, even though Content-Length: 0 already says there's nothing to read.

To Reproduce

import asyncio
from aiohttp import multipart
from aiohttp.streams import StreamReader
from aiohttp.base_protocol import BaseProtocol


async def main():
    boundary = b"boundary42"
    body = (
        b"--" + boundary + b"\r\n"
        b"Content-Type: text/plain\r\n"
        b"Content-Length: 0\r\n"
        b"\r\n"
        b"\r\n"
        b"--" + boundary + b"--\r\n"
    )

    loop = asyncio.get_event_loop()
    protocol = BaseProtocol(loop=loop)
    stream = StreamReader(protocol, 2**16, loop=loop)
    stream.feed_data(body)
    stream.feed_eof()

    reader = multipart.MultipartReader(
        {"Content-Type": f'multipart/mixed; boundary="{boundary.decode()}"'},
        stream,
    )
    part = await reader.next()
    await part.read_chunk(4)  # any size smaller than len(boundary) + 2


asyncio.run(main())

AssertionError: Chunk size must be greater or equal than boundary length + 2

Expected behavior

read_chunk() on a part with Content-Length: 0 returns b"" immediately regardless of the requested size, the same as it does for a nonzero Content-Length.

Logs/tracebacks

Traceback (most recent call last):
  File "repro.py", line 30, in <module>
    asyncio.run(main())
  File "repro.py", line 27, in main
    await part.read_chunk(4)
  File ".../aiohttp/multipart.py", line 392, in read_chunk
    fresh = await self._read_chunk_from_stream(want)
  File ".../aiohttp/multipart.py", line 451, in _read_chunk_from_stream
    size >= self._boundary_len
AssertionError: Chunk size must be greater or equal than boundary length + 2

Python Version

$ python --version
Python 3.14.3

aiohttp Version

$ python -m pip show aiohttp
Name: aiohttp
Version: 3.14.3

multidict Version

N/A (not exercised by this bug)

yarl Version

N/A (not exercised by this bug)

OS

macOS (platform independent, pure-Python code path)

Related component

Server, Client

Additional context

Reading through BodyPartReader, every other use of self._length in multipart.py correctly checks is not None (_align_base64_chunk, _read_chunk_from_length's assert). read_chunk's if self._length: is the odd one out.

Code of Conduct

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