Identify built-in generation preparation failures

internal/generation/evidence.go

@@ -10,6 +10,8 @@ import (
 // GenerationEvidence contains invocation-local request evidence without raw
 // provider bodies, headers, credentials, or generated content.
 type GenerationEvidence struct {
+	// PreparationFailed identifies a built-in request rejected before attempts.
+	PreparationFailed bool
 	// InitialEndpoint is the frozen built-in request URL, if prepared.
 	// Injected generation leaves it unknown.
 	InitialEndpoint string