fix: CSS var() truncation in advanced styles panel (#5378)

[niatalwalkar]

Description

This PR fixes a UI bug by preserving and displaying the complete CSS variable (without truncating the second parameter) in both the input field and the dropdown suggestion list under Display.
Closes #5378

Steps for reproduction

Before this fix (current behavior):

1. Open an existing project or create a new one.
2. Select an element and navigate to Style → Advanced → + → Add new CSS Property and choose display.
3. Set display to: var(--anchor-display, none).
4. The displayed value is truncated and the none appears to be missing under the Advanced section, even though it’s saved internally.

After this fix (expected behavior):

1. Follow the same steps as above.
2. The advanced property and the Display dropdown both consistently show var(--anchor-display, none) without truncation.

Code Review

• hi @kof, I need you to do
  • conceptual review (architecture, feature-correctness)
  • detailed review (read every line)
  • test it on preview

Before requesting a review

• made a self-review
• added inline comments where things may be not obvious (the "why", not "what")

Before merging

• tested locally and on preview environment (preview dev login: 0000)
• updated test cases document
• added tests
• if any new env variables are added, added them to .env file

[niatalwalkar]

Hi! It looks like the CI failed because GitHub doesn’t allow workflows from forked PRs to post comments. This seems unrelated to my code changes. Please let me know if I need to do anything on my side!