worker_client: tests, and stop resurrecting a closed worker

The layer between the app and idalib had no tests, which is awkward: it is
where failures are silent. A worker that dies during startup, a socket that
drops mid-call, two UI threads sharing one socket -- none of those look like
bugs from outside, they look like the TUI hanging or showing stale data.

None of it needs IDA. WorkerClient spawns whatever _WORKER_PY points at, so the
suite points it at a fake speaking the same length-prefixed pickle and tells it
to misbehave on demand: die at startup, never bind, drop the connection, fail a
tool, take its time. 40 checks in the --fast tier.

Two things the tests found:

call() reconnects when _sock is None, which is what makes a dropped socket
recoverable -- but it made an explicitly CLOSED client resurrect too, spawning a
whole new idalib worker to serve one stray call (verified: pid 1066961 ->
1066962). close() runs on teardown and on binary-switch while @work threads are
still in flight, so quitting during a decompile could leave a fresh process
re-opening the .i64 we had just released, which is the wedging hazard. A closed
client now refuses; connect() still revives it, which is all _reconnect needs
(it builds a new client anyway).

connect() polled on a flat 0.2s sleep, so every caller paid a fifth of a second
even when the worker was ready in milliseconds -- a seeded .i64, a small binary.
Backs off from 5ms instead.

786 checks, 144.6s; --fast is 297 in 3.3s.

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