Add HTTP multipart transport for GraphQL subscriptions

[magicmark]

Fix #463 -- implement support for Subscriptions over HTTP/1 multipart responses

See: https://www.apollographql.com/docs/graphos/routing/operations/subscriptions/multipart-protocol

This PR was initially authored by Claude Code, but I have since made significant changes.

fwiw in the process of implementing this, I also stumbled upon an edge case which possibly requires an upstream fix to aiohttp: aio-libs/aiohttp#11857

---

Claude's slop PR description fwiw:

Implements the multipart subscription protocol for receiving streaming subscription updates over HTTP as an alternative to WebSocket transports. This protocol is implemented by Apollo GraphOS Router and other compatible servers, and is particularly useful when WebSocket connections are not available or blocked by infrastructure.

The transport handles multipart/mixed responses with heartbeat support and proper error handling for both GraphQL and transport-level errors. It requires servers to support the multipart subscription protocol - requests that don't receive a multipart response will fail with a clear error message.

🤖 Generated with Claude Code

[magicmark]

cc @patrick91 ;)

[leszekhanusz]

Thanks for your work on this!

So do you know if there is a public backend somewhere supporting this protocol?

[magicmark]

So do you know if there is a public backend somewhere supporting this protocol?

not that i know of - but i'll start doing that now, i can throw something up on a lambda worker

[leszekhanusz]

not that i know of - but i'll start doing that now, i can throw something up on a lambda worker

Great!

[codecov]

Codecov Report

❌ Patch coverage is 99.17355% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.97%. Comparing base (7fb869a) to head (364f340).
⚠️ Report is 45 commits behind head on master.

Files with missing lines	Patch %	Lines
gql/transport/http_multipart_transport.py	99.17%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #574      +/-   ##
===========================================
- Coverage   100.00%   99.97%   -0.03%     
===========================================
  Files           38       41       +3     
  Lines         2908     3361     +453     
===========================================
+ Hits          2908     3360     +452     
- Misses           0        1       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[leszekhanusz]

For the linting issues, you should run make check to fix the code. See CONTRIBUTING.md

[magicmark]

@leszekhanusz https://github.com/magicmark/gql-book-server/ :)

publicly accessible here here: https://gql-book-server.fly.dev/graphql

(I was too dumb to get multipart + chunked transfer encoding working via a lambda so just deploying on the cheapest fly.io machine possible)

[magicmark]

@leszekhanusz ready for review!

[leszekhanusz]

@magicmark Thanks for the work and the backend. Do you want me to push it over the finish line by fixing the few remaining issues?