Closes #5371

Summary

Replacing a selection that ends inside a code block's language box (typing, pasting or Backspace) corrupted that code block: the rest of the language was copied into the paragraph where the selection starts while the code block kept its whole language, and the code block lost its language box, so its language could no longer be edited until the document was reopened (undo did not bring it back). Inside a list item, the blocks between the start of the selection and the code block were also left in place. Shift+↓ from a paragraph above lands the end of the selection there.

A cross-block cut merges endBlock.text.substring(endOffset) into the start block and calls removeBlocks(startBlock, endBlock). With the language input as endBlock, the merged text is the rest of the language, and pruneAfterBranch removes the language input node itself (a content leaf, so no json op is sent). muyajs never removed a language input.

New resolveCutEnd() in packages/muya/src/clipboard/cut.ts: a cut that ends in a language line stops before that code block. Nothing from the language joins the start block, the blocks between the start and the code block are removed as usual (up to the last leaf before the code block), and the code block keeps its code and language input and loses only the selected start of its language. The #918 path (a cut that starts in a language line) resolves its end the same way, so a cut from one language line into another collapses the first code block and trims the second.

Steps to reproduce

Checked in the released MarkText 0.20.0-rc.2 (macOS) and in a desktop build of this branch, with real mouse and keyboard input.

1. Open a document with:

   intro
   
   - item
   
     ```js
     code
     ```
   
     tail

2. Click between in and tro.
3. Press Shift+↓ twice (the selection ends at the start of the language box), then Backspace.
4. Click in the code line and look for the language box above it.

Top-level variant (Intro paragraph / code block / Outro, select Int|ro paragraph → j|s, type X): develop saves IntXs with the code block still ```js and no language box; this PR saves IntX and a code block with language s.

Type of change

• Bug fix (non-breaking, fixes an issue)
• New feature (non-breaking, adds functionality)
• Breaking change (causes existing functionality to change)
• Documentation update

Test plan

• New unit tests in clipboard/__tests__/cutIntoLanguageInput5371.spec.ts (4 cases): end inside the language (top level), end at the start of a nested code block's language (blocks before it removed), whole language selected, and from one language line into another. Each checks the json state against the block tree and that every code block still starts with its language input. All 4 fail on develop.
• New e2e spec editing/cut-into-language-input-5371.spec.ts: Backspace with the real keyboard, then types into the kept language box. Fails on develop, passes with the fix (3/3 repeated).
• Existing Code block no longer be input #918 / Unexpected error with message Cannot insert into out of bounds index #4903 cut specs still pass.
• Real desktop app (Playwright _electron, real click + Shift+↓ ×2 + Backspace): the table above, on the packaged 0.20.0-rc.2 and on a build:unpack of this branch.
• muya, after rebasing onto fix(muya): collapse only the nested code block when a cut starts in its language line (#5368) #5370: vitest run 237 files / 1675 tests; lint:types, eslint on the changed files and check-circular pass; the Unexpected error with message Cannot insert into out of bounds index #4903 / [Bug] Cutting from a nested code block's language box replaces the whole list or quote #5368 / [Bug] Cutting a selection that ends in a code block's language box duplicates text and removes the language box #5371 cut e2e specs pass.
• Manually tested on: macOS

Notes for reviewers

Rebased onto #5370 (#5368), which changed the same lines in collapseLanguageInputCut: the paragraph now replaces startBlock.getAnchor() (from #5370), and the blocks up to the resolved cut end are removed only when they lie outside that code block (this PR).

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

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