Adapt HealthChat to upstream v26.10.0

Follow-ups needed after rebasing the HealthChat series from v26.07.0 onto
v26.10.0 that span several of the replayed commits:

- Rename the five PreviewParameterProvider subclasses in features/healthchat
  to the *PreviewParam suffix now enforced by Konsist.
- Rename MemberDirectoryEvents and HealthChatProfileSectionEvents to the
  *Event suffix used by every upstream sealed interface.
- Import FakeStartDMAction from features/startchat/test, where it lives now.
- ListItem lost its headlineContent parameter in favour of content.
- HomeView gained onAddAccountClick and onSwitchToAccount, and onRoomClick
  now carries an EventId; update HomeViewBackHandlingTest accordingly.
- Restore upstream's codecov-action pin (v7.1.1) that the CI squash carried
  back to v7.0.0.
- Gate the new pr-checks.yml on the upstream repository: this fork does not
  use the PR- label convention.
- Add the 26.10.0 section to HEALTHCHAT_CHANGES.md.

HEALTHCHAT_CHANGES.md

@@ -6,6 +6,18 @@ top section matching the released version — the heading must have the exact
 form `## Changes in <version> (<date>)` and the whole section must stay under
 500 characters. See `.claude/skills/releasing-healthchat/SKILL.md`.
 
+## Changes in 26.10.0 (2026-10-19)
+
+🎉 Neue Funktionen
+
+- Nachrichtensuche: Chats durchsuchen, auch in verschlüsselten Räumen.
+- Umfragen mit Mehrfachauswahl.
+
+🙌 Verbesserungen
+
+- Basis aktualisiert auf Element X 26.10.0 mit zahlreichen Fehlerbehebungen.
+- Einstellungen neu gegliedert in Konto- und App-Einstellungen.
+
 ## Changes in 26.08.0 (2026-08-06)
 
 🎉 Neue Funktionen