The second half of leadership. A standby that refuses to place but still holds
an agent's stream is worse than useless: commands travel down the stream the
agent opened, so the leader's placements fail with "no agent connected" for a
node that is plainly connected.
So a standby refuses the stream, before reading the opening message, and says
why. And an agent takes a list rather than an address, trying each in turn — a
standby refusing, a stopped control plane refusing the connection and a crashed
one never answering are the same thing from a node, and the answer to all three
is somewhere else.
`server` and `servers` are both accepted, with `server` first when both are
given: every agent configuration written before there could be a second control
plane names one address, and an operator adding a standby expects the one
already working to stay preferred. With a single address the behaviour is
exactly what it was, including the backoff — which is skipped only while there
is another address that has not just been tried, because the delay exists to
stop hammering one control plane and the next one has not been hammered.
Removed a check that asked whether one field was empty, which is how the first
run of these tests failed: the list was populated and `server` was not, and an
agent configured entirely correctly was refused by its own validation.
The tests run a real agent against a real service over h2c. The first attempt
used a plain HTTP server, and the refusal arrived as an HTTP/2 framing error
that said nothing about why — which is exactly what an operator would have seen.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
