Fix RTE blocks stylesheet loading to match BlockGrid/BlockList behavior

[Copilot]

RTE blocks required both Stylesheet and Stylesheets properties to load styles, while BlockGrid and BlockList only needed Stylesheets. Root cause: RTE was reading from context settings instead of using API endpoints.

Changes

Backend

• Added /preview/rte/stylesheets and /preview/rte/stylesheet (deprecated) endpoints to BlockPreviewApiController
• Reuses existing GetStylesheetPaths() method which merges both properties

Frontend

• Added getRteStylesheets() to BlockPreviewService with type definitions
• Refactored rich-text-preview.custom-view.element.ts to fetch via API like Grid/List
• Extracted stylesheet loading to #loadStylesheets() helper with error handling

Example

// Before: Read from context, only worked with Stylesheet property
this.observe(this.#blockPreviewContext?.settings, (settings) => {
    if (settings?.richText?.stylesheet) { /* ... */ }
});

// After: Fetch via API, works with either/both properties
const { data } = await BlockPreviewService.getRteStylesheets({
    query: { documentTypeUnique, nodeKey }
});

Now works with:

• Stylesheets: ["/path/to/styles.css"] ✅
• Stylesheet: "/path/to/styles.css" ✅ (backward compatible)
• Both properties ✅

Warning

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>RTE blocks - Stylesheets not loading</issue_title>
<issue_description>### Which Umbraco.Community.BlockPreview version are you using?

5.2.0

Which Umbraco version are you using? For example: 10.4.0 - don't just write v10

17.10

Bug summary

Stylesheets are not loading, even though the path is correct, you need both Stylesheet and Stylesheets properties to be set for it to work

options.RichText = new()
{
Enabled = true,
Stylesheet = "/Static/dist/app.bundle.css",
Stylesheets = ["/Static/dist/app.bundle.css"],
ViewLocations =
[
"/Views/Partials/richtext/Components/{0}.cshtml"
]
}

This problem doesn't occur on Blockgrid or BlocList.

Steps to reproduce

Create a RTE block

Setup Stylesheets OR Stylesheet property

No styles are are loaded.

Setup Stylesheet AND Stylesheets property instead

Styles are loaded.

Expected result / actual result

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes RTE blocks - Stylesheets not loading #225

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