Migrate UI cancel error to tagged error (#27112)

packages/opencode/test/cli/error.test.ts

@@ -1,6 +1,7 @@
 import { describe, expect, test } from "bun:test"
 import { AccountTransportError } from "../../src/account/schema"
 import { FormatError } from "../../src/cli/error"
+import { UI } from "../../src/cli/ui"
 
 describe("cli.error", () => {
   test("formats account transport errors clearly", () => {
@@ -15,4 +16,8 @@ describe("cli.error", () => {
     expect(formatted).toContain("This failed before the server returned an HTTP response.")
     expect(formatted).toContain("Check your network, proxy, or VPN configuration and try again.")
   })
+
+  test("formats cancelled UI errors as empty output", () => {
+    expect(FormatError(new UI.CancelledError())).toBe("")
+  })
 })