feat: added subprocess test client

[aranvir]

Description

This introduces new helper functions to create sync and async test clients that can be used to test SSE endpoints with infinite generators as outlined in #3654 and https://github.com/orgs/litestar-org/discussions/3547.

The litestar cli is run in a subprocess to start a web app instance with a random port. The helper function then sets up a basic sync or async client.

The test code and doc example show how this can be used to test an SSE endpoint with an infinite generator.

Overview:

• Adds subprocess_async_client and subprocess_sync_client
• Tests both clients
• Adds general documentation. Adds docs example for subprocess_async_client

Closes

Closes #3654

[aranvir]

Tests are failing because the redis fixture does not seem to work correctly. Tbh, added that as a best guess, so would need some help with fixing this. Alternatively, I just replace the test with a regular "does it connect and return the right thing" kind of test and ditch the SSE testing.

No idea why doc generation is failing.

[JacobCoffee]

Tests are failing because the redis fixture does not seem to work correctly. Tbh, added that as a best guess, so would need some help with fixing this. Alternatively, I just replace the test with a regular "does it connect and return the right thing" kind of test and ditch the SSE testing.

cc: @litestar-org/maintainers thought?