The latest hardening pass on PR #47838 still left `↓ See below.` in the
transcript after healthy final delivery. This change moves Mattermost
preview finalization into one helper that only clears the draft after a
confirmed normal final send, keeps in-place finalization for safe
single-final turns, and preserves the preview when final delivery fails.
Constraint: Keep the fix scoped to the Mattermost preview state machine already under review in PR #47838
Rejected: Keep rewriting the preview to `↓ See below.` after successful normal sends | leaves transcript noise on healthy turns
Rejected: Delete the preview before attempting the real final send | risks losing all visible output when delivery fails
Confidence: high
Scope-risk: narrow
Directive: Successful normal finals must leave either the finalized preview or the real final post, never an extra placeholder note
Tested: `pnpm test extensions/mattermost/src/mattermost/monitor.test.ts`; `pnpm test extensions/mattermost/src/mattermost/draft-stream.test.ts`; `pnpm lint -- extensions/mattermost/src/mattermost/monitor.ts extensions/mattermost/src/mattermost/monitor.test.ts`; `pnpm build`
Not-tested: Fresh Mattermost smoke on the PR branch itself
Related: PR #47838
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