Blocks: Preserves source of unrecognized blocks inside of Code Editor

Following the work of #38923 where we started preserving the source
content for blocks that the editor fails to recognize, this patch
uses that information in the Code Editor to ensure that we don't
accidentally corrupt or lose content when making edits there.

Previously the `serializeBlock` function, which generates the HTML
for the Code Editor, would try to re-generated the saved content
for blocks that we know are invalid. Because we know the input is
invalid we can guarantee that the output will have even more problems.

Now that `serializeBlock` function is aware of the source content for
a block, and if the block is marked invalid _and_ that source exists
it will pass that along to the output and skip the processing and
re-generation of the saved content. This ensures that errors don't
cascade and that unrelated edits to other blocks from the Code Editor
won't destroy content in the marked-invalid blocks.

packages/blocks/src/api/serializer.js

@@ -24,8 +24,11 @@ import {
 	getFreeformContentHandlerName,
 	getUnregisteredTypeHandlerName,
 } from './registration';
+import { serializeRawBlock } from './parser/serialize-raw-block';
 import { isUnmodifiedDefaultBlock, normalizeBlockType } from './utils';
 
+/** @typedef {import('./parser').WPBlock} WPBlock */
+
 /**
  * @typedef {Object} WPBlockSerializationOptions Serialization Options.
  *
@@ -337,12 +340,16 @@ export function getCommentDelimitedContent(
  * Returns the content of a block, including comment delimiters, determining
  * serialized attributes and content form from the current state of the block.
  *
- * @param {Object}                      block   Block instance.
+ * @param {WPBlock}                      block   Block instance.
  * @param {WPBlockSerializationOptions} options Serialization options.
  *
  * @return {string} Serialized block.
  */
 export function serializeBlock( block, { isInnerBlocks = false } = {} ) {
+	if ( ! block.isValid && block.__unstableBlockSource ) {
+		return serializeRawBlock( block.__unstableBlockSource );
+	}
+
 	const blockName = block.name;
 	const saveContent = getBlockInnerHTML( block );