fix(server): skip defaultValue null check for relation/morph fields on update (#21875)

## Description

Updating any metadata property (e.g. `description`, `label`) of an
existing **non-nullable RELATION** field fails with:

```
INVALID_FIELD_INPUT: Default value cannot be null for non-nullable fields
```

A relation field has no literal `defaultValue` (it's always `null`), so
the update-path validator rejects every required relation. **Creating**
the same field is fine — only **updates** fail.

This also blocks any incremental app re-sync (`yarn twenty dev --once`)
whose diff touches a required relation field.

## Fix

Added a guard in
`FlatFieldMetadataValidatorService.validateFlatFieldMetadataUpdate()`
using the already-imported `isMorphOrRelationUniversalFlatFieldMetadata`
utility to skip the `defaultValue === null` check for relation/morph
field types:

```diff
 if (
+  !isMorphOrRelationUniversalFlatFieldMetadata(
+    flatFieldMetadataToValidate,
+  ) &&
   flatFieldMetadataToValidate.isNullable === false &&
   flatFieldMetadataToValidate.defaultValue === null
 ) {
```

### Why this works:
- Relation fields represent foreign key relationships, not columns with
literal defaults
- The same guard is already used at line 144 in the same method for
relation-specific validation
- The create path (`validateFlatFieldMetadataCreation`) never had this
check, which is why creation always worked
- No new imports needed — `isMorphOrRelationUniversalFlatFieldMetadata`
is already imported on line 14

## Verification
- `npx nx build twenty-server` ✅ compiles successfully

Fixes #21751

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---------

Co-authored-by: Charles Bochet <charles@twenty.com>
Co-authored-by: prastoin <paul@twenty.com>

packages/twenty-shared/src/types/FieldMetadataDefaultValue.ts

@@ -1,5 +1,5 @@
 import { type LinkMetadata } from '@/types/composite-types/links.composite-type';
-import { type FieldMetadataType } from '@/types/FieldMetadataType';
+import { FieldMetadataType } from '@/types/FieldMetadataType';
 import { type IsExactly } from '@/types/IsExactly';
 
 export const fieldMetadataDefaultValueFunctionName = {
@@ -111,3 +111,17 @@ export type FieldMetadataDefaultValue<
     : T extends keyof FieldMetadataDefaultValueMapping
       ? FieldMetadataDefaultValueMapping[T]
       : never | null;
+
+export const FIELD_METADATA_TYPES_WITHOUT_DEFAULT_VALUE = [
+  FieldMetadataType.RELATION,
+  FieldMetadataType.MORPH_RELATION,
+  FieldMetadataType.FILES,
+  FieldMetadataType.TS_VECTOR,
+] as const satisfies FieldMetadataType[];
+
+export const isFieldMetadataTypeWithDefaultValue = (
+  type: FieldMetadataType,
+): boolean =>
+  !(
+    FIELD_METADATA_TYPES_WITHOUT_DEFAULT_VALUE as readonly FieldMetadataType[]
+  ).includes(type);

packages/twenty-shared/src/types/__tests__/field-metadata-default-value-classification.type-test.ts

@@ -0,0 +1,19 @@
+import { type Equal, type Expect } from '@/testing';
+import {
+  type FieldMetadataDefaultValueMapping,
+  type FIELD_METADATA_TYPES_WITHOUT_DEFAULT_VALUE,
+} from '@/types/FieldMetadataDefaultValue';
+import { type FieldMetadataType } from '@/types/FieldMetadataType';
+
+type ClassifiedFieldMetadataType =
+  | keyof FieldMetadataDefaultValueMapping
+  | (typeof FIELD_METADATA_TYPES_WITHOUT_DEFAULT_VALUE)[number];
+
+type Unclassified = Exclude<FieldMetadataType, ClassifiedFieldMetadataType>;
+type ExtraClassified = Exclude<ClassifiedFieldMetadataType, FieldMetadataType>;
+
+// oxlint-disable-next-line unused-imports/no-unused-vars
+type Assertions = [
+  Expect<Equal<Unclassified, never>>,
+  Expect<Equal<ExtraClassified, never>>,
+];

packages/twenty-shared/src/types/index.ts

@@ -92,7 +92,11 @@ export type {
   FieldMetadataDefaultValueForAnyType,
   FieldMetadataDefaultValue,
 } from './FieldMetadataDefaultValue';
-export { fieldMetadataDefaultValueFunctionName } from './FieldMetadataDefaultValue';
+export {
+  fieldMetadataDefaultValueFunctionName,
+  FIELD_METADATA_TYPES_WITHOUT_DEFAULT_VALUE,
+  isFieldMetadataTypeWithDefaultValue,
+} from './FieldMetadataDefaultValue';
 export type { FieldMetadataMultiItemSettings } from './FieldMetadataMultiItemSettings';
 export { FieldMetadataSettingsOnClickAction } from './FieldMetadataMultiItemSettings';
 export type {