test(mdviewer): add code block editing and checkbox integration tests

Add Code Block Editing tests: ArrowDown exit, Shift+Enter exit, Enter
creates new line within block, non-last-line navigation stays in block,
last-block creates new paragraph, CM→viewer content sync, and language
change sync. Add checkbox-test.md and code-block-test.md fixtures.
Add __clickCheckboxForTest helper in bridge.js.

Author: abose

src-mdviewer/to-create-tests.md

@@ -1,26 +1,5 @@
 # Markdown Viewer/Editor — Integration Tests To Create
 
-## Slash Menu (/ command)
-- [x] Slash menu appears when typing / at start of line
-- [x] Escape dismisses slash menu
-- [x] Typing after / filters menu items (e.g. /image shows Image items)
-- [ ] Slash menu positioning (visual — needs real viewport)
-- [ ] Arrow down/up scrolls selected item into view
-- [ ] Selected item wraps around (last → first, first → last)
-- [ ] Slash menu works at bottom of a long scrolled document
-
-## Code Block Editing
-- [ ] ArrowDown on last line of code block exits to paragraph below
-- [ ] ArrowDown on last line creates new `<p>` if none exists below
-- [ ] ArrowDown on last line moves to existing next sibling if present
-- [ ] ArrowDown on non-last line navigates normally within code block
-- [ ] Shift+Enter on last line of code block exits to paragraph below
-- [ ] Shift+Enter on non-last line does NOT exit (normal behavior)
-- [ ] Enter inside code block creates new line (not exit)
-- [ ] Code block exit syncs new paragraph to CM source
-- [ ] Intermediate code blocks: Shift+Enter exits on last line
-- [ ] Last code block in document: ArrowDown creates `<p>` and exits
-
 ## Table Editing
 - [ ] Clearing a table cell with backspace produces valid markdown (no broken pipe rows)
 - [ ] Empty table cell renders as `|  |` in markdown source

test/spec/LiveDevelopment-Markdown-test-files/code-block-test.md

@@ -0,0 +1,20 @@
+# Code Block Test
+
+Some text before code blocks.
+
+```javascript
+function hello() {
+    console.log("hello");
+    return true;
+}
+```
+
+A paragraph between code blocks.
+
+```python
+def greet():
+    print("hello")
+    return True
+```
+
+Final paragraph after code blocks.