feat(FR-2594): add E2E test for SSO sToken login flow

[nowgnuesLee]

Resolves #6757 (FR-2594)

Summary

• Add Playwright E2E test for SSO sToken-based login flow (e2e/auth/sso-stoken-login.spec.ts)
• Verifies that users can authenticate via an SSO sToken URL and land on the correct page
• Covers the scenario originally fixed in FR-2574

Test plan

• Run pnpm run test:e2e against a live Backend.AI cluster with SSO configured
• Verify test passes for valid sToken login
• Verify test covers error/invalid token cases

[nowgnuesLee]

• feat(FR-2594): add E2E test for SSO sToken login flow #6758 👈 (View in Graphite)
• main

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[Copilot]

Pull request overview

Adds a new Playwright E2E spec to validate the root-URL /?sToken=... SSO login flow and its session persistence behavior, expanding Auth-related regression coverage in the WebUI E2E suite.

Changes:

• Adds an E2E test verifying sToken auto-login redirects to /start and removes sToken from the URL.
• Adds an E2E test verifying the authenticated session persists across a page refresh.
• Preconfigures config.toml during the test to ensure apiEndpoint is present for the orchestration flow.