Keep Duo mention started note as durable record

What does this MR do and why?

Implements the UX improvement requested in #602991: when a Duo mention workflow completes, we remove the transient "started" (thinking) system note and append a "View session" link to the result message, so the session stays referenced close to the outcome instead of leaving a leftover system note in the thread.

Before: The "started … session N and will let you know when it's finished" note stayed in the thread after completion (or behaved inconsistently across surfaces), and on the bot path it could disappear/reorder unexpectedly on live update.

After: While the flow runs, the started note shows an animated loader. Once the result lands, the started (thinking) note is destroyed and the reply note is posted with a [View session](url) link appended at the bottom. This works for both the @GitLabDuo bot path and the Duo Developer service-account path on MRs.

This direction was agreed in the review threads (see discussion): show the thinking message, remove it once the result lands, and append the session details to the result message (starting small with a simple "View session" link, taking design inspiration from the Code Review path as a follow-up).

Changes

Backend

• duo_mention_started builds SystemNoteMetadata (action: duo_mention_started) and Notes::NoteDuoMetadata (workflow_id, namespace_id) on the created note, so the workflow link is queryable without parsing the note body.
• deliver_result/deliver_error load the started note once (load_started_note), reuse it for both the workflow lookup and destruction (no duplicate query), then destroy it and post the reply. destroy_started_note rescues ActiveRecord::RecordNotDestroyed and tracks the exception so a destroy failure never blocks the reply.
• deliver_result appends a [View session](url) link to the reply via append_session_link. A blank result message still posts the session link rather than being swallowed.
• Session wording drops the word "mention" → "session N".
• NoteEntity exposes duo_session_status (the workflow's status_name, not the raw enum), gated on the duo_mention_started action, used by the frontend to route rendering and drive the spinner.

Frontend

• Routing of the styled Duo system note is centralised in a single shouldRenderAsDuoSystemNote(note) helper (~/notes/utils.js), used by both discussion_notes.vue and notes_app.vue. It routes on the presence of duo_session_status (covers both the bot and service-account mention notes) and keeps the duo_code_review_bot author check for the separate Duo Code Review progress note (which has no duo_session_status).
• Removal of the started (thinking) note on reply is centralised in a single findStartedNoteForReply(notes, discussions) helper (~/notes/utils.js), called from both stores/actions.js and legacy_notes/actions.js (kept symmetric, in fetchUpdatedNotes). The helper only removes a started note when an incoming reply note shares a discussion that actually contains a started note (duo_session_status present), so ordinary comments no longer trigger removal.
• duo_code_review_system_note.vue: the loader is shown and animated only while the session is in progress (created/running) and hidden once terminal (finished/failed/stopped) or waiting; absent status falls back to animated for notes created before this field existed.

Out of scope / follow-ups

• Issues (work items) use a separate GraphQL notes stack and do not render the styled card/spinner. The backend durability (persist + destroy-on-completion) works there, but the visual unification is a follow-up.
• A richer styled "View session" treatment (matching the Code Review design) instead of the simple markdown link.
• The shared DuoCodeReviewSystemNote avatar styling differs slightly from a normal note avatar (pre-existing).
• MR-vs-issue system-note unification and any public API exposure.

References

• #602991
• gitlab-org#22067
• Main work item: #601705

How to set up and validate locally

Prereqs: a DAP-onboarded project with the mention foundational flow(s) enabled and a :mention service-account trigger, and the Duo Workflow executor running in GDK.

# Duo Developer @mention path
Feature.enable(:ai_use_messaging_adapter_for_mentions)

1. Duo Developer @mention (MR thread): comment @<duo-developer-service-account> … on an MR.
   • A started note appears — "started session N and will let you know when it's finished" — with an animated loader.
   • When the flow completes, the started (thinking) note is removed and the reply note is posted with a [View session](url) link appended at the bottom — without reloading.
2. @GitLabDuo MR mention (requires duo_foundational_flows_enabled + the code-review catalog item enabled on the project): comment @GitLabDuo … on an MR.
   • Same started → removed-on-completion behavior, and the started note no longer disappears or jumps position when unrelated comments arrive on live update.
3. Failure path: trigger a flow error (e.g. by cancelling the session) → the started note is removed and an error reply is posted.
4. Try the same on an issue (the UI will look a bit different there but this is parked for a follow-up).

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