test(agentsfleetd): prove the claim path frees what it loaded when it refuses

Claiming a non-runnable fleet returns after the row's owned copies are taken,
so that early return is the path every errdefer in `claimFleet` exists for —
including the one added with the fleet's name. It had no test, so the chain was
asserted by reading rather than by running.

`std.testing.allocator` is the assertion: it fails on any byte the early return
orphans. That is the only proof the errdefers are correct; "looks right" is not.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>