Add Playwright E2E tests for sample applications

[akunzai]

• Create E2E test project with Playwright and xUnit
• Add Page Objects for Home, Login, and Keycloak login pages
• Implement tests for AspNetCoreSample, AspNetCoreMvcSample, BlazorSample, AspNetCoreIdentitySample, and AspNetCoreReactSample
• Update DevContainers to use Debian-based image for Playwright support
• Add Playwright browser dependencies installation
• Create e2e-tests.yml workflow for CI/CD
• Update solution file to include new test project

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 70.41%. Comparing base (2d615a7) to head (3cf4d30). :warning: Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #818   +/-   ##
=======================================
  Coverage   70.41%   70.41%           
=======================================
  Files          42       42           
  Lines        1048     1048           
  Branches      136      136           
=======================================
  Hits          738      738           
  Misses        248      248           
  Partials       62       62           

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