fix: serialize the Text (HTML) XBlock's include_theme setting [WIP]

[rpenido]

Description

XBlockSerializer writes a hard-coded allowlist of fields to OLX. The Text (HTML) XBlock's new opt-in include_theme setting is not in that allowlist, so it survives in the block's in-memory state after a save but is silently dropped from the exported OLX. Reopening the unit loses the setting. This PR adds it to the serializer.

The reader (HtmlBlock.parse_xml_new_runtime) is generic and already picks the attribute up, so writing it here is all that is needed for a full round-trip.

User roles impacted: Course Author (the setting now survives save/export). No learner-visible change.

Supporting information

Principal PR: openedx/xblocks-core#308 — adds the include_theme field to the XBlock and the JavaScript that honours it. This PR is the openedx-platform half: without it the checkbox appears to work and then reverts on save.

Testing instructions

• See feat: add an opt-in MFE theme to the Text (HTML) XBlock [WIP] xblocks-core#308 for instructions.

Other information

• Depends on: feat: add an opt-in MFE theme to the Text (HTML) XBlock [WIP] xblocks-core#308
• Used by: feat: add include_theme toggle to the Text XBlock [WIP] frontend-app-authoring#3271

Deadline

None.

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Private ref: FAL-4394

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[rpenido]

include_theme is from xblocks-core, but explicitly referenced here
We could try to use a more generic solution, but it may cause other fields to be serialized too

[samuelallan72]

@rpenido is it possible to make the html block declare the field? Or is this just an unnecessary comment? I see plenty of other special handling for the html block here, so I don't think this needs an extra comment?

[rpenido]

I don't think we need to declare it. The comment is about the "include_theme" in block.fields guard to avoid an exception when using the legacy HTML xblock from here (xmodule/html_block.py).

openedx-platform/xmodule/html_block.py

Lines 550 to 559 in 4b5aff6

	if not settings.USE_EXTRACTED_HTML_BLOCK:
	warnings.warn(
	"The built-in `xmodule.html_block` HtmlBlock implementation is deprecated. "
	"To fix this warning, enable `USE_EXTRACTED_HTML_BLOCK` (set it to True) to use "
	"`xblocks_contrib.html.HtmlBlock` instead. "
	"Support for the built-in implementation, and the `USE_EXTRACTED_HTML_BLOCK` setting, "
	"will be removed in Willow.",
	DeprecationWarning,
	stacklevel=2,
	)

We probably don't need this comment. Removed here: d5fbdf2

[samuelallan72]

Thanks for clarifying :) @rpenido