Expose sent message identifiers in workflow send-email step output (#22520)

## Context

First step toward thread-continuity / follow-up email steps in workflows
(email sequences). The outbound send pipeline already knows the sent
email's RFC-822 Message-ID, the provider thread id, and the persisted
message/thread records — but none of it was surfaced in the send-email
step output, so a later step had no way to reference the email that was
sent.

## What changed

- `saveMessagesWithinTransaction` also returns a `messageExternalId →
messageThreadId` map, and `saveMessagesAndEnqueueContactCreation`
returns the message/thread id maps (both other call sites ignore the
return value)
- `SentMessagePersistenceService.persistSentMessage` and
`SendEmailService.persistSentMessage` return the persisted `{ messageId,
messageThreadId }` (`undefined` when persistence is skipped or fails —
sending still succeeds)
- `SendEmailTool` result now includes `headerMessageId`,
`threadExternalId`, `messageId` and `messageThreadId`
- SEND_EMAIL step output schema (server + frontend) declares
`headerMessageId`/`messageId`/`messageThreadId` so they show up in the
variable picker; DRAFT_EMAIL keeps its success-only schema since draft
creation returns no identifiers yet

This already enables manual thread continuity today: wire
`{{sendEmailStep.headerMessageId}}` into a later email step's
In-Reply-To advanced field — the composer resolves the References chain
and provider thread from it.

## Tests

- New `send-email-tool.spec.ts` covering identifiers in the result,
persistence disabled, and persistence failure
- Extended save-messages spec with the new map, updated frontend
`computeStepOutputSchema` tests

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packages/twenty-front/src/modules/workflow/workflow-variables/utils/generate/__tests__/computeStepOutputSchema.test.ts

@@ -425,12 +425,48 @@ describe('computeStepOutputSchema', () => {
   });
 
   describe('SEND_EMAIL step', () => {
-    it('should return success boolean schema', () => {
+    it('should return success and sent message identifier schema', () => {
       const result = computeStepOutputSchema({
         step: { type: 'SEND_EMAIL', settings: {} } as any,
         objectMetadataItems: [],
       });
 
+      expect(result).toEqual({
+        success: {
+          isLeaf: true,
+          type: FieldMetadataType.BOOLEAN,
+          label: 'Success',
+          value: true,
+        },
+        headerMessageId: {
+          isLeaf: true,
+          type: FieldMetadataType.TEXT,
+          label: 'Message-ID header',
+          value: '',
+        },
+        messageId: {
+          isLeaf: true,
+          type: FieldMetadataType.TEXT,
+          label: 'Message record ID',
+          value: '',
+        },
+        messageThreadId: {
+          isLeaf: true,
+          type: FieldMetadataType.TEXT,
+          label: 'Message thread ID',
+          value: '',
+        },
+      });
+    });
+  });
+
+  describe('DRAFT_EMAIL step', () => {
+    it('should return success boolean schema', () => {
+      const result = computeStepOutputSchema({
+        step: { type: 'DRAFT_EMAIL', settings: {} } as any,
+        objectMetadataItems: [],
+      });
+
       expect(result).toEqual({
         success: {
           isLeaf: true,

packages/twenty-front/src/modules/workflow/workflow-variables/utils/generate/computeStepOutputSchema.ts

@@ -218,7 +218,35 @@ export const computeStepOutputSchema = ({
       return generateFormOutputSchema(formFields, objectMetadataItems);
     }
 
-    case 'SEND_EMAIL':
+    case 'SEND_EMAIL': {
+      return {
+        success: {
+          isLeaf: true,
+          type: FieldMetadataType.BOOLEAN,
+          label: 'Success',
+          value: true,
+        },
+        headerMessageId: {
+          isLeaf: true,
+          type: FieldMetadataType.TEXT,
+          label: 'Message-ID header',
+          value: '',
+        },
+        messageId: {
+          isLeaf: true,
+          type: FieldMetadataType.TEXT,
+          label: 'Message record ID',
+          value: '',
+        },
+        messageThreadId: {
+          isLeaf: true,
+          type: FieldMetadataType.TEXT,
+          label: 'Message thread ID',
+          value: '',
+        },
+      };
+    }
+
     case 'DRAFT_EMAIL': {
       return {
         success: {