[DEFECT] Language Variable Selector Incorrectly Shown in JSON Field Editor

Problem Statement

The JSON field editor in the new content editor displays a language variable selector button that allows users to insert Velocity syntax (e.g., $text.get('label.yes')), but the backend does not support Velocity evaluation in JSON fields. This creates a confusing user experience where:

1. Users see the feature: The UI displays a language variable selector button in JSON field editor
2. Save fails without quotes: When users insert unquoted Velocity syntax, they get validation error: "Invalid JSON field provided"
3. Saves but doesn't evaluate: When wrapped in quotes, JSON saves successfully but Velocity syntax is stored as literal string and NOT evaluated when retrieved via Content API

Steps to Reproduce

Detailed steps:

1. Navigate to Content Types → Edit a content type with a JSON field
2. Create or edit content using this content type
3. In the JSON field editor, observe the language variable selector button (globe icon) in the top-right corner
4. Click the language variable selector button
5. Select a language variable (e.g., $text.get('label.yes'))
6. The Velocity syntax is inserted into the JSON field
7. Attempt to save the content
8. Result:
   • If unquoted: Error "Invalid JSON field provided"
   • If quoted: Saves successfully, but when retrieved via Content API, the Velocity syntax is returned as literal string (not evaluated)

Acceptance Criteria

• Language variable selector button should NOT be displayed for JSON fields, OR
• If displayed, Velocity syntax should be evaluated when content is retrieved via Content API

dotCMS Version

Latest evergreen

Severity

Low - Minor issue or cosmetic

Links

https://dotcms.freshdesk.com/a/tickets/35089
https://dotcms.slack.com/archives/C08F8NZRK9S/p1768933183860019
https://dotcms.slack.com/archives/C08F8NZRK9S/p1768933616813999

Start at the new content editor's JSON field editor and trace the language variable selector entry point, then reproduce the unquoted and quoted cases described in the issue. Done means the selector is absent for JSON fields, or the inserted Velocity syntax is evaluated when the content is retrieved through the Content API.