Fix SDK plan on non installed apps (#22805)

# Context

`yarn twenty plan` fails when the app has never been installed in the
target workspace:

```
Sync failed with error: Application "f5ce204f-..." is not installed in workspace "10f39a9d-...". Install it first.
Hint: run `yarn twenty dev --once` to register the app in this workspace, then retry.
```

This forces developers to apply before they can plan, which defeats the
purpose of `plan`. Planning a not-yet-installed app is well defined: the
from-state is empty, so the plan is simply "create everything".

## Why it failed

The dry-run sync required the application row to exist in two places:

1. `ApplicationSyncService.synchronizeFromManifest` threw
`APP_NOT_INSTALLED` when the app row was missing, because the dry-run
needs an owner `FlatApplication` to anchor the from → to metadata diff.
2.
`WorkspaceMigrationFlatEntityMapsService.computeAllInvolvedApplicationIds`
threw when the owner app id was absent from `flatApplicationMaps`, even
though the only hard dependency of a build is the twenty standard
application.

`apply` never hit this because it registers the app row as a side effect
before syncing (and even swallows this exact error on its pre-apply
plan).

# What this PR does

Keeps `plan` strictly read-only, no registration or app row is created:

- **`application-sync.service.ts`**: on dry-run, resolve the owner to
the installed application when it exists (unchanged behavior), otherwise
build a virtual, non-persisted `FlatApplication` from the manifest. Its
freshly generated id matches no existing metadata, so the from-state
slice resolves to empty and every manifest entity shows up as a create.
- **`workspace-migration-flat-entity-maps.service.ts`**: relax the guard
so only the twenty standard application is required. A missing owner app
just contributes an empty from-slice instead of throwing. Installed apps
take the exact same path as before (`applicationId` defined → identical
behavior).

packages/twenty-server/src/engine/workspace-manager/workspace-migration/services/workspace-migration-flat-entity-maps.service.ts

@@ -247,14 +247,16 @@ export class WorkspaceMigrationFlatEntityMapsService {
         TWENTY_STANDARD_APPLICATION.universalIdentifier
       ];
 
-    if (!isDefined(twentyStandardApplicationId) || !isDefined(applicationId)) {
+    if (!isDefined(twentyStandardApplicationId)) {
       throw new FlatEntityMapsException(
-        'Application to build and its dependent application not found',
+        'Twenty standard application not found in workspace',
         FlatEntityMapsExceptionCode.ENTITY_NOT_FOUND,
       );
     }
 
-    applicationIds.add(applicationId);
+    if (isDefined(applicationId)) {
+      applicationIds.add(applicationId);
+    }
 
     const isBuildingTwentyStandardApplication =
       applicationUniversalIdentifier ===