Fail a run step whose answer is over the bus's value limit, naming its size (issue 1921, review round 1)

Claude-Session: https://claude.ai/code/session_01NcbgfFSnn24M3GcqttoU7i

modules/mesh-build-go/cmd/mesh-build-go/run.go

@@ -44,6 +44,13 @@ func (m *Module) run(ctx context.Context, fetch buildstep.StepFunc, a delivery.S
 		return nil, err
 	}
 	ans.Built, ans.Key = built, key
+	// The answer is written on the step key as one value: one over the bus's value limit (the input's bound) fails the
+	// step, naming its size, before it is written.
+	if raw, err := json.Marshal(ans); err != nil {
+		return nil, err
+	} else if len(raw) > delivery.RunInputBound {
+		return nil, fmt.Errorf("the checking program's answer is %d bytes, over the bound of %d bytes a step key holds", len(raw), delivery.RunInputBound)
+	}
 	return ans, nil
 }
 

modules/mesh-build-go/cmd/mesh-build-go/run_test.go

@@ -95,6 +95,13 @@ func TestAKeptProgramIsRunOverItsHeadsAndInput(t *testing.T) {
 	if ws < 0 || heads < ws || !strings.Contains(line, " "+delivery.RunHeadsEnv+" "+argv[heads+1]+" ") {
 		t.Errorf("the heads are not bound read-only over the workspace: %s", line)
 	}
+	// An answer over the bus's value limit fails the step, naming its size.
+	m.Run = func(context.Context, []string, string, ...string) forgekey.Ran {
+		return forgekey.Ran{Status: 0, Stdout: `"` + strings.Repeat("x", delivery.RunInputBound) + `"`}
+	}
+	if _, err := m.run(context.Background(), fetch, a); err == nil || !strings.Contains(err.Error(), "over the bound") {
+		t.Errorf("an answer over the bound: %v", err)
+	}
 	m.Inside, m.Run = true, StepRunner(t.TempDir())
 	// An input other than the one named is refused, naming it.
 	m.ReadInput = func(string, string) (json.RawMessage, error) { return json.RawMessage(`{"other":1}`), nil }