Stub unarchiveSnapshot in stale AgentManager test doubles

Commit 25252d1b8 added AgentManager.unarchiveSnapshot, now called from
the shared unarchiveAgentState helper. The hand-rolled manager doubles in
the session-workspaces and import-sessions suites were not updated, so the
refresh/import RPC paths hit an unstubbed method and failed. Add the
method to those doubles; no production change.

packages/server/src/server/session.workspaces.test.ts

@@ -543,6 +543,7 @@ function createSessionForWorkspaceTests(
         getAgent: () => null,
         archiveAgent: async () => ({ archivedAt: new Date().toISOString() }),
         archiveSnapshot: async () => ({}),
+        unarchiveSnapshot: async () => true,
         clearAgentAttention: async () => {},
         notifyAgentState: () => {},
       }),