fix: pick ephemeral port once per agent

[ikonst]

Fixes spurious low level errors in the form of "socket hang up" when rapidly testing.

By creating a listening server just once per agent, we can allow callers to strategize by reusing the agent and avoiding (a) the setup overheard, and (b) spurious failure due to the ephemeral port being random on every agent construction (previously, on every request).

Fixes #844.

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