Show validation errors in elicitation dialog when Enter is pressed

[dgageot]

Previously, pressing Enter on an elicitation dialog with invalid fields would silently do nothing, making users think the Enter key wasn't working.

Now the dialog:

• Highlights invalid field labels in red
• Shows descriptive error messages below each invalid field
• Focuses the first field with an error
• Clears errors when the user modifies a field

This provides clear feedback when validation fails, such as:

• 'This field is required' for empty required fields
• 'Must be a valid number' for invalid numeric input
• 'Must be at least N' for minimum value violations
• 'Must be a valid email address' for email format errors

Assisted-By: cagent