Markdown: a second formatting pass removes nested emphasis

Prettier 3.9.6

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--parser markdown

Input:

**a _**b**_ c**

Output:

**a _**b**\_ c**

Expected output:

**a _**b**_ c**

Why?

Prettier itself produces this input when formatting **a ***b*** c**. That first pass preserves CommonMark rendering. Formatting again removes the emphasis on b and introduces literal underscores. Equivalent formatting that preserves the original emphasis is fine.

Also reproduced on development commit 891accaaab6dab0fbb5eba6748fbb1f5ac4809b5. Related #17143 changed delimiter styling; this example still fails in 3.9.6.

GPT-6 Astra assisted; I manually reviewed this report.