Fix #1140: Improve Field Creator error messaging for permission failures

[bishal-selegic]

Describe your changes

• Modified the field deployment logic in addon/field-creator.js to separate field creation status from permission assignment status.
• Introduced a Warning state (slds-theme_warning / info icon) for when a field is successfully created but assigning permissions to a restricted profile fails.
• Replaced temporary strings with proper user-facing messages: "Field created successfully, but permission assignment failed".
• Improved the dynamic switching of SLDS themes based on the error status.

Issue ticket number and link

Fixes #1140

Checklist before requesting a review

• I have read and understand the Contributions section
• My PR relates to an existing issue or feature request and I discussed it with maintainer
• I used SLDS style and limit the usage of custom CSS
• I have performed a self-review of my code
• I ran the unit tests and my PR does not break any tests
• I documented the changes I've made on the CHANGES.md and followed actual conventions
• I added a new section on how-to.md (optional)

[bishal-selegic]

Hi @tprouvot , I’ve implemented the fix for this issue. Would you mind taking a look when you have time? Thanks!