Add support for rendering Unicode combining characters (diacritics) in
the terminal's batched text runs.

- Add append_zero_width_chars() to handle combining marks
- Integrate zero-width chars into all batching code paths
- Update cell extras tracking logic
- Add test for combining character rendering

Fixes display of é, ñ, ô and other diacritics.

Closes #39525

Release Notes:

- Fixed: NFD/NFKD normalized text (e.g., é as e + ◌́) not rendering in
integrated terminal

Before:

<img width="874" height="688" alt="SCR-20251004-udnj"
src="https://github.com/user-attachments/assets/8d9f9c9f-dac4-4382-92c2-8b6c1d817abd"
/>

After:

<img width="873" height="686" alt="SCR-20251004-ulsw"
src="https://github.com/user-attachments/assets/fbd5cdc7-fdd6-44dc-8b05-cc425644f1a0"
/>