Authenticate xdist StatusDB connections

[janmrow]

Generate a per-session token on the xdist controller and pass it to workers through workerinput.

StatusDB now requires a successful handshake before processing commands and rejects clients with an invalid token.

Tests cover the authentication check and passing the connection details to workers.

[icemac]

Reviewed the handshake against the branch (suite also run under xdist: 175 passed). The change itself is sound — the token is assigned before run_server starts, the with conn, suppress(...) nesting order is right, failures_db is always a ServerStatusDB where .token is read, the token survives execnet serialization, and both new tests fail before the change and pass after.

Three things on the error paths, though; the first two chain into a session hang under xdist.

— Comment created by Claude