e2e: Added Posit AI auth workflow to assistant tests

[jonvanausdeln]

This adds a sign-in test for Posit AI provider in Positron Assistant. This enables other test to use Posit AI as a provider.

• Add e2e test infrastructure for Posit AI OAuth device code flow authentication
• Add posit-ai model provider support to the assistant page object with methods to handle OAuth login via a separate browser
• Add test for Posit AI sign-in and sign-out functionality
• Add Posit auth domains (login.posit.cloud, login.staging.posit.cloud) to trusted domains to prevent confirmation dialogs during tests

The credentials are pulled from 1Password. For now, they are using mine, but I will update this soon to a test/service account set.

@jmcphers , just checking it's okay to add PositAI login URL to the trusted list.

QA Notes

@:assistant @:win @:web

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical @:assistant @:win @:web

^{readme  valid tags}

[testlabauto]

Nice!
LGTM!