feat(messaging): sync draft emails and edit them in the thread composer (#22178)

Stop excluding drafts from sync across all three providers (Gmail DRAFT
label, Microsoft/IMAP Drafts folder) and add an isDraft boolean field on
Message so drafts are queryable by the API and AI agents.

Drafts render in the thread with a Draft tag; clicking one opens the
existing reply composer pre-filled with the draft's recipients, subject
and body, and Send reuses the existing send-email flow.

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Co-authored-by: Félix Malfait <felix.malfait@gmail.com>

packages/twenty-server/src/engine/workspace-manager/twenty-standard-application/utils/field-metadata/compute-message-standard-flat-field-metadata.util.ts

@@ -415,4 +415,24 @@ export const buildMessageStandardFlatFieldMetadatas = ({
     twentyStandardApplicationId,
     now,
   }),
+  isDraft: createStandardFieldFlatMetadata({
+    objectName,
+    workspaceId,
+    context: {
+      fieldName: 'isDraft',
+      type: FieldMetadataType.BOOLEAN,
+      label: i18nLabel(msg`Is draft`),
+      description: i18nLabel(
+        msg`Whether this message is an unsent draft synced from the provider`,
+      ),
+      icon: 'IconPencil',
+      isNullable: false,
+      isUIEditable: false,
+      defaultValue: false,
+    },
+    standardObjectMetadataRelatedEntityIds,
+    dependencyFlatEntityMaps,
+    twentyStandardApplicationId,
+    now,
+  }),
 });