Carry evidence through built-in generation retries

internal/observation/usage_accumulator_test.go

@@ -0,0 +1,27 @@
+package observation
+
+import (
+	"testing"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+func TestUsageAccumulatorPreservesPresenceAndNoAttemptSemantics(t *testing.T) {
+	var empty UsageAccumulator
+	zero := empty.Observed()
+	if !zero.PromptTokens.Known || !zero.PromptTokens.Complete || zero.PromptTokens.Value != 0 ||
+		!zero.CacheWriteTokens.Known || !zero.CacheWriteTokens.Complete || zero.CacheWriteTokens.Value != 0 {
+		t.Fatalf("zero-attempt usage = %+v", zero)
+	}
+
+	var usage UsageAccumulator
+	usage.Add(domain.TokenUsage{PromptTokens: 0}, &domain.TokenUsagePresence{PromptTokens: true})
+	usage.Add(domain.TokenUsage{PromptTokens: 3}, &domain.TokenUsagePresence{PromptTokens: true})
+	observed := usage.Observed()
+	if observed.PromptTokens.Value != 3 || !observed.PromptTokens.Known || !observed.PromptTokens.Complete {
+		t.Fatalf("reported prompt usage = %+v", observed.PromptTokens)
+	}
+	if observed.CompletionTokens.Known || observed.CompletionTokens.Complete {
+		t.Fatalf("unreported completion usage = %+v", observed.CompletionTokens)
+	}
+}