Add GatewayFailure: the provider not answering is not a tool failing

terminationFor sent every gateway error that was not Refused or Timeout
to ToolFailure, because the enum had nowhere else to put it. On
2026-09-22 that was 131 of 318 production runs, and not one of them was
a tool failing: 56 were retired model ids, 25 an exhausted Anthropic
balance, 45 Groq's free-tier rate limit -- the only one still happening.
An operator reading the termination column saw "a tool is broken" for
two weeks while the actual answer was "we are not paying for capacity".

GatewayFailure is the seventh termination. Rate limited, request
rejected, credential refused and unreachable land there; Refused and
Deadline keep their own reasons; a non-gateway error is still the tool
layer's. A delegation whose subagent died at the gateway now carries
that reason up to the parent instead of reading as a tool call that
failed.

Migration 000016 widens the CHECK that 000006 chose precisely so this
would be a migration rather than an ALTER TYPE. Its down folds any
GatewayFailure rows back to ToolFailure BEFORE narrowing the constraint,
which is the order that works; verified up, down and up again on a
scratch database. Existing rows are left as they are -- the trajectory
entries still carry the gateway.* code for anyone reclassifying history.

The surface wording is the one termination where "try again" is honest
advice, since the dominant cause clears within a minute.

Full suite run against a real database, including the tests that skip
without one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PJvibeSc1JYXjatankqM1g

go-api/internal/httpserver/runs.go

@@ -205,7 +205,7 @@ func buildRunResponse(res *runtime.ExecutionResult) runResponse {
 // reached its budget before finishing" is accurate and means nothing to
 // somebody who has never heard of a token budget.
 //
-// Every one of the six is spelled out. A default that said "something went
+// Every one of the seven is spelled out. A default that said "something went
 // wrong" would be the place where a Refused run and a ToolFailure became
 // indistinguishable to the person best placed to tell us which it was.
 func terminationMessage(t runtime.Termination) string {
@@ -224,6 +224,12 @@ func terminationMessage(t runtime.Termination) string {
 			"Nothing was changed."
 	case runtime.TerminationRefused:
 		return "The agent declined to answer this one."
+	case runtime.TerminationGatewayFailure:
+		// The one termination where "try again" is honest advice: the
+		// dominant cause is a rate limit that clears within a minute, and
+		// nothing about the question itself was the problem.
+		return "The model behind this agent did not answer — usually it is busy. " +
+			"Wait a minute and ask again. Nothing was changed."
 	default:
 		return "The agent did not finish."
 	}