Summary

Backspace at the start of a setext heading turned the heading into a paragraph in the model, but let the browser's own Backspace run as well. The browser then merged the new paragraph's element into the block above and removed it, so the paragraph stayed in the block tree with a detached element. With a bullet list above the heading, pressing Enter twice at the end of the list threw NotFoundError: Failed to execute 'insertBefore' on 'Node' from ScrollPage.insertBefore ← ScrollPage.insertAfter ← ParagraphContent._enterInListItem — the #5035 crash.

This PR fixes that one trigger and closes #5035. The two other triggers found while investigating it have their own issues, which track the remaining work:

• [Bug] Typing over a triple-clicked line above a task list or table detaches it, and Enter on a list then throws "Failed to execute 'insertBefore'" #5414 — typing over a triple-clicked line above a task list or table;
• [Bug] Cmd+Backspace across two blocks detaches the second block, and Enter on a list then throws "Failed to execute 'insertBefore'" #5415 — Cmd+Backspace (and other edits that never reach the keydown handler) across two blocks.

Steps to reproduce

1. Open a file containing:

   - one
   - two
   
   Heading
   ---

2. Click at the very start of Heading and press Backspace.
3. Click at the end of the last bullet line, press Enter, then press Enter again.

• Expected: Backspace turns the heading into a paragraph in place, and the second Enter leaves an empty paragraph below the list.
• Actual (v0.20.0-rc.1, v0.20.0-rc.3, develop): after step 2 Heading jumps onto the end of two on screen while the document keeps both (saved as - one / - twoHeading / blank line / Heading), and the second Enter opens the "Unexpected renderer process error" dialog. The same happens with an === underline.

Root cause

SetextHeadingContent.backspaceHandler converts the heading to a paragraph at offset 0 but never calls event.preventDefault(). The ATX heading, paragraph and thematic break handlers all do. The browser's default Backspace then merges the new paragraph's element into the block above.

Fix

Call event.preventDefault() before convertToParagraph, as the ATX heading handler does.

Test plan

• Unit src/block/content/setextHeadingContent/__tests__/backspaceHandler.spec.ts: Backspace at offset 0 is default-prevented and yields a paragraph; Backspace mid-text still goes to the input handler. The first case fails with the source change reverted.
• e2e e2e/tests/editing/setext-heading-backspace-5035.spec.ts (Chromium): click at the start of a setext heading below a bullet list, Backspace, then Enter twice at the end of the list. With the source change reverted it fails with detached blocks and the insertBefore NotFoundError; with it, it passes.
• pnpm -C packages/muya test (1786 tests), pnpm -C packages/muya lint:types, eslint on the changed files (the e2e spec with --no-ignore).
• Desktop app: the steps above crash on packaged v0.20.0-rc.1 and v0.20.0-rc.3 and on a develop build, and are clean on a build that carried this change (checked earlier together with the fixes now split into [Bug] Typing over a triple-clicked line above a task list or table detaches it, and Enter on a list then throws "Failed to execute 'insertBefore'" #5414 and [Bug] Cmd+Backspace across two blocks detaches the second block, and Enter on a list then throws "Failed to execute 'insertBefore'" #5415).

Fixes #5035

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