Summary

tests/shared/test_ws.py used the subprocess + TCP port pattern that races under pytest-xdist: a worker allocates a port with socket.bind(0), releases it, then spawns a uvicorn subprocess hoping to rebind. Between release and rebind, another worker can claim the port — observed in CI as the WS client connecting to another worker's HTTP/SSE server and getting a 403 Forbidden on the upgrade (job 63414508256).

What changed

Three tests → in-memory Client(server) transport (no network, no subprocess, no race):

• test_ws_client_happy_request_and_response
• test_ws_client_exception_handling
• test_ws_client_timeout

These verify transport-agnostic MCP semantics (read_resource happy path, MCPError propagation, session recovery after client-side timeout). None of them were testing WebSocket framing.

One test kept as real-TCP smoke test → test_ws_client_basic_connection

Uses a new run_uvicorn_in_thread helper that binds port=0 atomically and reads the actual port back from the server's bound socket — the OS holds the port from bind to shutdown, eliminating the race window. This test alone provides 100% coverage of src/mcp/client/websocket.py.

Cleanup

• Removed handle_list_tools / handle_call_tool (never exercised by any test)
• Removed # pragma: no cover from handle_read_resource (now runs in-process)
• Replaced anyio.sleep(2.0) with anyio.sleep_forever() for the slow-resource handler — semantically clearer that it blocks until cancellation

Overlap with MAX-158

MAX-158 adds the same run_uvicorn_in_thread helper to fix the port race across many test files. Whichever PR lands first, the other will have a trivial rebase (the helper implementation is identical). This PR keeps wait_for_server in test_helpers.py since other test files on main still use it.

Test Plan

• ./scripts/test — full suite + 100% coverage ✓
• pytest tests/shared/test_ws.py -n 4 --flake-finder --flake-runs=5 — 20 parallel runs, 0 flakes ✓
• pyright + ruff ✓

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