fix: use is string not is mapping in content-field fallback

`is mapping` is Jinja2-only — Nunjucks doesn't have it, so the test
always evaluated false, falling through to the raw content object and
rendering as [object Object].

Fix: `_contentFv.text or (_contentFv if _contentFv is string else "")`
correctly handles both object fallback (extracts .text) and string
fallback (validation re-render).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/patch-endpoint-posts-content-field-object.mjs

@@ -24,7 +24,7 @@ const newSnippet = `{#- ${marker} -#}
 {%- set _contentFv = fieldData("content").value -%}
 {{ textarea({
   name: "content",
-  value: properties.content.text or (_contentFv.text if _contentFv is mapping else _contentFv) or "",`;
+  value: properties.content.text or _contentFv.text or (_contentFv if _contentFv is string else ""),`;
 
 async function exists(p) {
   try { await access(p); return true; } catch { return false; }