Cancel subscription sign-in requests when leaving agent setup (#19610)

Back during pending subscription sign-in could accept a late response and
reopen agent setup. Abort pending reads, release abandoned sessions, and
ignore stale responses. Wait for cleanup before starting a replacement,
and preserve sessions belonging to another account or environment.

Cover deferred resume/start/prompt responses for both providers, local
login cancellation, quick reselection, and browser request abort. Stabilize
Runner fixtures and normalization assertions exposed by integration CI.

Verified: 483 focused UI tests, six onboarding browser cases, UI typecheck
and token gates, Runner fixture checks, and the complete CI run on
221f2e3eb7. Greptile scored 5/5;
all review threads are resolved. Earlier failures and recovery evidence
remain in the PR description.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

tests/runner-e2e/report.test.ts

@@ -391,6 +391,7 @@ describe("runner E2E report aggregation", () => {
           GITHUB_SERVER_URL: "https://github.com",
           GITHUB_REPOSITORY: "paperclipai/paperclip",
           GITHUB_RUN_ID: "123456",
+          GITHUB_RUN_ATTEMPT: "1",
           PAPERCLIP_RUNNER_E2E_HISTORY_PUBLIC_BASE_URL:
             "https://reports.example.test/",
           PAPERCLIP_RUNNER_E2E_HISTORY_PREFIX: "/runner-e2e/",