test: skip real-transport tests on Windows (they require os.fork)

[K4bain]

The server fixture in both real-transport test modules spawns the FastAPI app in a subprocess via multiprocessing.get_context("fork"):

ctx = multiprocessing.get_context("fork")

os.fork() does not exist on Windows, so every test in test_sse_real_transport.py and test_http_real_transport.py errors with ValueError: cannot find context for 'fork' when the suite is run on Windows (fork was chosen over spawn to avoid pickling the FastAPI app, so there is no drop-in substitute).

This PR adds a module-level pytestmark = pytest.mark.skipif(os.name == "nt", ...) to both modules, with a comment explaining why. Behavior on POSIX (including CI) is unchanged — the tests still run there. Locally on Windows the suite now reports 69 passed, 20 skipped instead of 20 errors.

[K4bain]

Closing for housekeeping - the repo had no reviewer engagement and we are re-approaching these areas through a narrower pipeline. Happy to reopen on request.