[Apps] Fix - app-synced object should be searchable (#19206)

## Summary

- **Make app-synced objects searchable**: `isSearchable` was hardcoded
to `false` and the `searchVector` field was missing the `GENERATED
ALWAYS AS (...)` expression, causing all records to have a `NULL` search
vector and be excluded from search results. Fixed by defaulting
`isSearchable` to `true` (configurable via the object manifest),
computing the `asExpression` from the label identifier field, and
allowing the update-field-action-handler to handle the `null` → defined
`asExpression` transition.
- **Make `isSearchable` updatable on an object**: The property had
`toCompare: false` in the entity properties configuration, so updates
via the API were silently ignored and never persisted. Fixed by setting
`toCompare: true`.

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

@@ -204,6 +204,8 @@ export { assertIsDefinedOrThrow } from './validation/assertIsDefinedOrThrow';
 export { isDefined } from './validation/isDefined';
 export { isEmptyObject } from './validation/isEmptyObject';
 export { isLabelIdentifierFieldMetadataTypes } from './validation/isLabelIdentifierFieldMetadataTypes';
+export type { SearchableFieldType } from './validation/isSearchableFieldType';
+export { isSearchableFieldType } from './validation/isSearchableFieldType';
 export { isValidLocale } from './validation/isValidLocale';
 export { isValidTwentySubdomain } from './validation/isValidTwentySubdomain';
 export { isValidUuid } from './validation/isValidUuid';