feat(service): record upstream model across all gateway paths

Propagate UpstreamModel through ForwardResult and OpenAIForwardResult in Anthropic direct, API-key passthrough, Bedrock, and OpenAI gateway flows. Extract optionalNonEqualStringPtr and optionalTrimmedStringPtr into usage_log_helpers.go. Store upstream_model only when it differs from the requested model.

Also introduces anthropicPassthroughForwardInput struct to reduce parameter count.

backend/internal/service/usage_log_helpers.go

@@ -0,0 +1,21 @@
+package service
+
+import "strings"
+
+func optionalTrimmedStringPtr(raw string) *string {
+	trimmed := strings.TrimSpace(raw)
+	if trimmed == "" {
+		return nil
+	}
+	return &trimmed
+}
+
+// optionalNonEqualStringPtr returns a pointer to value if it is non-empty and
+// differs from compare; otherwise nil. Used to store upstream_model only when
+// it differs from the requested model.
+func optionalNonEqualStringPtr(value, compare string) *string {
+	if value == "" || value == compare {
+		return nil
+	}
+	return &value
+}