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/runtime/budget.go

@@ -22,13 +22,26 @@ const (
 	TerminationConfirmationPending Termination = "ConfirmationPending"
 	TerminationToolFailure         Termination = "ToolFailure"
 	TerminationRefused             Termination = "Refused"
+
+	// GatewayFailure is the model provider failing to answer at all: rate
+	// limited, rejected the request, refused the credential, or unreachable.
+	// Added 2026-09-22 because until then every one of those was recorded as
+	// ToolFailure, and 131 of 318 production runs read as "a tool is broken"
+	// when no tool had failed — 45 of them were Groq's free-tier rate limit,
+	// which is a capacity decision, not a bug. The two are different questions
+	// to an operator ("what did we break" versus "what are we not paying
+	// for"), and an enum that could not tell them apart hid the answer for
+	// two weeks. Refused and Deadline keep their own reasons; this is the
+	// rest of the gateway's vocabulary.
+	TerminationGatewayFailure Termination = "GatewayFailure"
 )
 
-// Valid reports whether t is one of the six.
+// Valid reports whether t is one of the seven.
 func (t Termination) Valid() bool {
 	switch t {
 	case TerminationCompleted, TerminationBudgetExceeded, TerminationDeadline,
-		TerminationConfirmationPending, TerminationToolFailure, TerminationRefused:
+		TerminationConfirmationPending, TerminationToolFailure, TerminationRefused,
+		TerminationGatewayFailure:
 		return true
 	}
 	return false