Name the homeserver's administrator when it cannot be reached

[hayaksi1]

Content

"We couldn't reach this homeserver" tells the user to contact an administrator without saying who
that is. The Matrix spec has a place for that answer:
GET /.well-known/matrix/support,
served by the delegating domain rather than the homeserver API, so it usually answers even when the
API does not — which is exactly the case in the report.

On the failure path only, the login flow now reads that file and adds the administrator's email
address, Matrix ID or support page to the dialog. An admin contact is preferred over the other roles,
an email address over a Matrix ID, and the support page is the last resort. A homeserver that
advertises nothing, or that cannot be reached at all, keeps today's text — the lookup never fails the
login, and it never runs on the success path.

Deliberately out of scope: raising the log level during authentication and offering the logs from the
dialog, the other half of manuroe's comment.

Motivation and context

Part of #4556. manuroe, reopening it as an enhancement: "And to really finish the work, we need to
add the admin details in this error popup as adverstised by GET /.well-known/matrix/support."

Tests

• HomeserverSupportContactProviderTest — role preference, email over Matrix ID, support page as the
  last resort, and nothing advertised.
• OnBoardingPresenterTest — an unreachable homeserver produces
  ChangeServerError.InvalidServer(supportContact = …) when one is advertised and
  ChangeServerError.InvalidServer() when not.
• Negative control: dropping the enrichment makes present - an unreachable homeserver reports the administrator it advertises fail.
• ./gradlew :features:login:impl:testDebugUnitTest :features:login:impl:ktlintCheck :features:login:impl:detekt — green.
• The dialog variant is covered by a new ChangeServerErrorPreviewParam value, appended last so the
  existing goldens keep their indices; the two new ones were recorded by CI.

Tested devices

• Physical
• Emulator
• OS version(s): unit tests and Composable previews

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[github-actions]

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[bmarty]

Thanks for the enhancement.

Since the Matrix specs is defining where the administrator can be reached, the change has to be located in the SDK first. Probably ClientBuildException.ServerUnreachable should carry the admin email if available.

[hayaksi1]

Moved the lookup into the SDK as suggested: matrix-org/matrix-rust-sdk#6924. I'll rework this PR on top of it once that lands.