`Reader` gets a test-only fixed source that counts its reads, and `State`
takes the agent's own list as a `fn` (`available_models` outside tests), so
`start` and `continue_` can be driven through `State::allow_model` without
a queue or a process-wide env change. The new tests pin that both calls
refuse a model outside the swarm list ahead of `reserve`, accept a model
the swarm lists but the local list lacks, and read nothing for a call
without a `model`.
Refs #5034 (S5)
f27818340d