fix(ssh): stop a failed worktree scan from publishing authoritative emptiness (#17833)

* fix(ssh): keep an unreadable worktree catalog from authorizing teardown

#14004: the relay's worktree-list fallback caught every failure and returned
`[]`, so `SshGitProvider.listWorktrees` resolved as a success with an empty
list. Downstream reconciliation treats a resolved listing as authoritative,
which reaches `teardownMissingWorktreeTerminalsBestEffort` and the
unregistered-worktree removal paths — a data-loss path from a failed scan.

- relay: the `-z`-unsupported fallback lane propagates its failure instead of
  swallowing it to `[]`.
- provider: an empty or malformed `git.listWorktrees` response is refused as
  `WorktreeCatalogUnavailableError`. A Git repo always lists its own checkout,
  so a zero-row listing can only be a scan that never answered — this is the
  mixed-version guard against relays that still swallow.
- `listRepoWorktrees`: an unreachable SSH host reports unavailable instead of
  an empty catalog.

#12661: `ssh:terminateSessions` now returns `{ terminated, unverifiable }`, so
an offline sweep that only tore down local transport cannot be mistaken for a
remote kill. The Manage-hosts toast warns instead of claiming success.

* chore(i18n): register the unreachable-terminal terminate message

src/preload/api/ssh-api.ts

@@ -9,7 +9,8 @@ import type {
   SshTarget,
   SshTargetAddResult,
   SshTargetCreateInput,
-  SshTargetUpdateInput
+  SshTargetUpdateInput,
+  SshTerminateSessionsResult
 } from '../../shared/ssh-types'
 import type { FilesystemPathFlavor } from '../../shared/filesystem-entry-types'
 
@@ -25,7 +26,7 @@ export type SshApi = {
   resolveConfigHost: (args: { alias: string }) => Promise<SshConfigHostResolution | null>
   connect: (args: { targetId: string }) => Promise<SshConnectionState | null>
   disconnect: (args: { targetId: string }) => Promise<void>
-  terminateSessions: (args: { targetId: string }) => Promise<void>
+  terminateSessions: (args: { targetId: string }) => Promise<SshTerminateSessionsResult>
   resetRelay: (args: { targetId: string }) => Promise<void>
   getState: (args: { targetId: string }) => Promise<SshConnectionState | null>
   needsPassphrasePrompt: (args: { targetId: string }) => Promise<boolean>