fix: prevent default object re-selection in relation field form (#17313)

Fixes #17111

### Problem
When creating a Relation field, after deselecting the pre-selected
default object (e.g., Company) and selecting a different object (e.g.,
Opportunity), both objects would end up being selected, showing "2
Objects" instead of just the newly selected one.

### Root Cause
The `initialMorphRelationsObjectMetadataIds` array was being recreated
on every component render, causing react-hook-form's `Controller` to
treat the `defaultValue` prop as a new value and re-apply it after user
changes.

### Solution
1. **Memoized the initial value**: Used `useMemo` to ensure
`initialMorphRelationsObjectMetadataIds` has a stable reference across
renders
2. **Added initialization guard**: Used `useEffect` with a `useRef` flag
to ensure the default value is only set once during component mount
3. **Maintained Controller defaultValue**: Kept the `defaultValue` prop
on the Controller, which now works correctly with the stable memoized
value

### Changes
- `SettingsDataModelFieldRelationForm.tsx`: Added memoization and
initialization guard to prevent default value re-application

---------

Co-authored-by: Etienne <45695613+etiennejouan@users.noreply.github.com>

packages/twenty-front/src/modules/ui/input/components/MultiSelectControl.tsx

@@ -20,6 +20,7 @@ type MultiSelectControlProps = Omit<SelectControlProps, 'selectedOption'> & {
   fixedIcon?: IconComponent;
   fixedText?: string;
   selectedOptions: MultiSelectOptionType[];
+  placeholderText?: string;
 };
 
 export const MultiSelectControl = ({
@@ -30,16 +31,22 @@ export const MultiSelectControl = ({
   selectSizeVariant,
   textAccent = 'default',
   hasRightElement,
+  placeholderText,
 }: MultiSelectControlProps) => {
   const theme = useTheme();
 
-  const firstSelectedOption = selectedOptions?.[0];
+  const firstSelectedOption = selectedOptions[0];
+  const hasSelection = selectedOptions.length > 0;
+
   return (
     <StyledControlContainer
       disabled={isDisabled}
-      hasIcon={isDefined(fixedIcon) || isDefined(firstSelectedOption?.Icon)}
+      hasIcon={
+        isDefined(fixedIcon) ||
+        (hasSelection && isDefined(firstSelectedOption?.Icon))
+      }
       selectSizeVariant={selectSizeVariant}
-      textAccent={textAccent}
+      textAccent={hasSelection ? textAccent : 'placeholder'}
       hasRightElement={hasRightElement}
     >
       {isDefined(fixedIcon) ? (
@@ -48,7 +55,7 @@ export const MultiSelectControl = ({
           size: theme.icon.size.md,
           stroke: theme.icon.stroke.sm,
         })
-      ) : isDefined(firstSelectedOption?.Icon) ? (
+      ) : hasSelection && isDefined(firstSelectedOption.Icon) ? (
         <firstSelectedOption.Icon
           color={isDisabled ? theme.font.color.light : theme.font.color.primary}
           size={theme.icon.size.md}
@@ -57,9 +64,11 @@ export const MultiSelectControl = ({
       ) : null}
       {isDefined(fixedText) ? (
         <OverflowingTextWithTooltip text={fixedText} />
-      ) : (
-        <OverflowingTextWithTooltip text={firstSelectedOption?.label ?? ''} />
-      )}
+      ) : hasSelection ? (
+        <OverflowingTextWithTooltip text={firstSelectedOption.label} />
+      ) : isDefined(placeholderText) ? (
+        <OverflowingTextWithTooltip text={placeholderText} />
+      ) : null}
 
       <StyledSelectControlIconChevronDown
         disabled={isDisabled}