Raise error on nested multipart parts in HTTP subscription responses

[Copilot]

The HTTP Multipart Transport was silently skipping nested multipart parts instead of treating them as protocol violations.

Changes

• gql/transport/http_multipart_transport.py: Replace continue with TransportProtocolError when isinstance(part, aiohttp.MultipartReader) is true
• tests/test_http_multipart_transport.py: Add test coverage for nested multipart error case

Behavior

# Before: nested multipart parts were silently ignored
if isinstance(part, aiohttp.MultipartReader):
    continue

# After: protocol violation raises immediately
if isinstance(part, aiohttp.MultipartReader):
    raise TransportProtocolError(
        "Received unexpected nested multipart part in GraphQL subscription response."
    )

Nested multipart is not part of the GraphQL subscription protocol spec. Raising an error makes protocol violations explicit rather than masking potential server misconfigurations.

Original prompt

On http_multipart_transport.py:231-231, actually this should throw an error

if isinstance(part, aiohttp.MultipartReader):
raise TransportProtocolError(
"Received unexpected nested multipart part in GraphQL subscription response."
)

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