STYLE/BUG: refine italic colour highlighting (matching all italics such as in bibtex references)

Our colour based italic system is a bit overapplied to some objects like `bibtex` references. Identified in https://github.com/QuantEcon/lecture-python.myst/pull/762

<img width="1028" height="588" alt="Image" src="https://github.com/user-attachments/assets/41f77b64-aff4-4847-849e-d9dc006805f1" />

Should be restricted to `markdown` emphasis.
