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.

.github/workflows/pr-checks.yml

@@ -10,6 +10,8 @@ permissions: {}
 
 jobs:
   pr-checks:
+    # Upstream-only: this fork does not use the PR- label convention.
+    if: github.repository == 'element-hq/element-x-android'
     name: PR Checks
     runs-on: ubuntu-latest
     timeout-minutes: 5