fix(core): keep backslashes before escaped HTML-like text

Text like \<details> exported as \\<details>, which the parser reads as
a literal backslash followed by raw HTML, so the text was lost. Double
the backslashes right before an escaped < so they load back unchanged.

Refs #3189

Author: jkugsiya

packages/core/src/api/exporters/markdown/htmlToMarkdown.ts

@@ -849,10 +849,14 @@ function serializeInlineContent(el: Element): string {
 /**
  * Escape `<` where the markdown parser would read raw HTML or an autolink, so
  * text like `<details>` loads back as text. A `<` before a space or digit
- * stays as is, as in `1 < 2`.
+ * stays as is, as in `1 < 2`. Backslashes right before an escaped `<` are
+ * doubled too, so `\<details>` doesn't become an escaped backslash plus HTML.
  */
 function escapeHtmlLikeText(text: string): string {
-  return text.replace(/<(?=[A-Za-z/!?])/g, "\\<");
+  return text.replace(
+    /(\\*)<(?=[A-Za-z/!?])/g,
+    (_, backslashes: string) => `${backslashes}${backslashes}\\<`,
+  );
 }
 
 /**

tests/src/unit/core/formatConversion/exportParseEquality/__snapshots__/markdown/markdown/paragraphWithBackslashBeforeHtmlLikeText.json

@@ -0,0 +1,19 @@
+[
+  {
+    "children": [],
+    "content": [
+      {
+        "styles": {},
+        "text": "Escaped \<details> and \\<b> stay text, as does C:\Users.",
+        "type": "text",
+      },
+    ],
+    "id": "1",
+    "props": {
+      "backgroundColor": "default",
+      "textAlignment": "left",
+      "textColor": "default",
+    },
+    "type": "paragraph",
+  },
+]

tests/src/unit/core/formatConversion/exportParseEquality/exportParseEqualityTestInstances.ts

@@ -513,6 +513,19 @@ export const exportParseEqualityTestInstancesMarkdown: TestInstance<
     },
     executeTest: testExportParseEqualityMarkdown,
   },
+  {
+    testCase: {
+      name: "markdown/paragraphWithBackslashBeforeHtmlLikeText",
+      content: [
+        {
+          type: "paragraph",
+          content:
+            "Escaped \\<details> and \\\\<b> stay text, as does C:\\Users.",
+        },
+      ],
+    },
+    executeTest: testExportParseEqualityMarkdown,
+  },
   {
     testCase: {
       name: "markdown/nestedLists",