Inline code marks allow other marks applied to their content

[b-kelly]

Describe the bug

Typing underscores into an inline code mark cause the text to be (incorrectly) italicized. The commonmark reference dingus shows that underscores inside a code span are not rendered as emphasis. In the following screenshot, notice that there are three code elements, with the center one wrapped in an emphasis element. This only happens in rich-text mode while using input commands (and not during round-trip markdown conversion).

To Reproduce
Steps to reproduce the behavior:

1. Switch to the rich-text editor
2. Open a new code block via any method (e.g. clicking the menu button)
3. Type the following text into the inline code mark: a_b_c
4. See that the underscores disappear and the middle character is italicized

Something else to note is that the rich-text mode seems to allow other marks inside the code mark (e.g. emphasis menu button is enabled). I'd imagine that merely fixing the schema would solve all of our issues here.

Expected behavior
Code marks should disallow all other marks applied to their content. This includes marks from any source - input commands, menu commands, etc.