feat(FN-1223): add remote node connection APIs to CentralCore

- Add a new NodeConnection utility that validates inputs, tests /api/health endpoints, normalizes URLs, and classifies connection failures
- Add CentralCore methods to test node connectivity and connect/register remote nodes, with duplicate-name checks and node:connection:test event emission
- Export NodeConnection types and APIs from @fusion/core for external consumers
- Add comprehensive unit coverage for NodeConnection and CentralCore connection flows, and increase dashboard assignment test setup timeout to reduce build/test flakiness

packages/dashboard/src/routes.test.ts

@@ -2029,7 +2029,7 @@ describe("PATCH /tasks/:id/assign and GET /agents/:id/tasks", () => {
       updateTask: vi.fn(),
       listTasks: vi.fn().mockResolvedValue([]),
     } as any);
-  });
+  }, 30_000);
 
   afterEach(() => {
     rmSync(tempDir, { recursive: true, force: true });