fix: detect UpstreamError with errors.As in isNetworkError

The stale-on-error path used a bare type assertion, which fails if the
UpstreamError is ever wrapped. Use errors.As so wrapping is handled.

Refs #68

internal/proxy/engine.go

@@ -5,6 +5,7 @@ import (
 	"crypto/sha256"
 	"encoding/hex"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"log/slog"
@@ -431,8 +432,6 @@ func (e *UpstreamError) Error() string { return fmt.Sprintf("upstream error: %v"
 func (e *UpstreamError) Unwrap() error { return e.Err }
 
 func isNetworkError(err error) bool {
-	if _, ok := err.(*UpstreamError); ok {
+	var ue *UpstreamError
-		return true
+	return errors.As(err, &ue)
-	}
-	return false
 }