## 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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Félix Malfait
9f4efa57ff
