Add Vitest and enhance testing coverage for Phoria Islands

[CMeeg]

This pull request establishes a test foundation for both the JavaScript and .NET parts of the project, and updates the CI pipeline to run these tests.

Test Infrastructure and Coverage

• Adds a new .NET test project, Phoria.Tests, with xUnit-based unit tests and integrates it into the solution and CI pipeline
• Introduces a smoke test for the framework-multiple e2e app using Vitest, ensuring SSR and island rendering are exercised, and adds a Vitest config for smoke tests
• Updates Nx configuration and root/package scripts to support and cache test and test:browser commands, wiring them into the monorepo test workflow

Continuous Integration Pipeline

• Expands the GitHub Actions CI workflow to run JavaScript unit tests, browser-mode component tests, .NET tests, and e2e smoke tests in appropriate jobs, ensuring full stack coverage

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Latest commit: 9119c09

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