This PR resolves intermittent test failures and flakiness in the CI matrix (specifically under pytest -n auto parallelism) caused by a time-of-check/time-of-use (TOCTOU) port race when allocating ports for test servers.

Closes #2704

Root Cause

Previously, test server port fixtures bound to 127.0.0.1:0 to find a free port, then closed the socket immediately to return the port number. Between this allocation and the server starting in a separate multiprocessing.Process to bind that port, another parallel worker could be assigned the same port. This caused two primary issues:

1. OSError: [Errno 98] Address already in use (failing to start the server)
2. Crossed/interleaved connections where a client connects to the wrong test server instance.

Solution

I converted the server processes to bind their socket ephemerally (127.0.0.1:0) and called listen() inside the child process itself. The socket remains open and in a listening state throughout the process lifecycle. The child process reports its dynamically allocated port back to the parent process using a unidirectional multiprocessing.Pipe.

• This fully closes the TOCTOU gap because the socket is never closed between check and use.
• It removes the need for wait_for_server polling (since the socket is already listening before the fixture yields).
• It remains fully cross-platform compatible as it does not require inheriting or pickling socket descriptors across process boundaries (only the pipe connection and integers are sent across process boundaries).

Affected Files Migrated

• tests/shared/test_streamable_http.py
• tests/shared/test_sse.py
• tests/server/test_sse_security.py
• tests/server/test_streamable_http_security.py
• tests/client/test_http_unicode.py

Other Cleanups

• Added a reusable running_server context manager in tests/test_helpers.py.
• Removed the unused wait_for_server helper from tests/test_helpers.py to maintain 100% test coverage.
• Updated affected test cases to consume these fixtures directly as URL strings, avoiding unnecessary server startups.

Verification

Automated Tests

uv run --frozen --no-default-groups --group dev pytest

• Result: 1271 passed (1172 passed, 98 skipped, 1 xfailed).

Linting & Formatting

uv run --frozen --no-default-groups --group dev ruff check .
uv run --frozen --no-default-groups --group dev pyright

• Result: 0 errors, 0 warnings.

Test Coverage

uv run --frozen --no-default-groups --group dev coverage report

• Result: 100.00% Coverage (19,675 statements, 0 missed).