fix: Keep the resource variable modal open when adding headers

[kof]

Description

Prevent accidental form submit in the resource variable modal when adding or removing headers and search params. The icon buttons in the resource editor were acting as submit buttons, which caused the new-variable modal to close unexpectedly instead of updating the resource fields. Updated the builder test-cases document with a manual regression flow.

Steps for reproduction

1. Select an element in a project
2. Open Settings for the element
3. Under data variables, create a new variable
4. Set the name
5. Select resource as the type
6. Enter a resource URL
7. Add a header
8. Expect the modal to stay open and a new header row to be added
9. Add a search param
10. Expect the modal to stay open and a new search param row to be added

resource-variable.mov

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