fix: WCEnd editor modal pre-loads correct colour and errno value

- Default colour now checks dataset.exitError instead of tagName === 'WC-END',
  so success (exit 0) END blocks default to blue, error END blocks to red
- _selectByCode now also sets the HTML value attribute via setAttribute(),
  ensuring cloneNode(true) preserves the formatted display value
- Add 4 regression tests for colour defaults, attribute sync, and clone value

helpers.js

@@ -382,7 +382,7 @@ function showBlockEditor(block) {
   body.innerHTML = "";
 
   /* Colour picker row; error blocks default to red, others to blue */
-  const defaultColor = block.classList.contains("error-path") || block.tagName === "WC-END" ? "#ef5350" : "#4a90d9";
+  const defaultColor = block.classList.contains("error-path") || block.dataset.exitError ? "#ef5350" : "#4a90d9";
   const colorRow = document.createElement("div");
   colorRow.className = "block-editor-color-row";
   colorRow.innerHTML =