Enhance TDD documentation with UI testing strategy

[ericelliott]

Added UI testing strategy guidelines and constraints.

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Note

Enhances the TDD guide with a clear UI testing strategy and minor cleanup.

• Adds UI testing strategy under Default Test Utils: pure tests for Redux actions/selectors, isolate side effects (see javascript/javascript-io-network-effects.mdc), use riteway/render for component markup, Playwright for real browser interactions, and avoid @testing-library/react.
• Minor formatting/content tweak at file end (/welcome).

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[Copilot]

Pull request overview

This pull request enhances the TDD documentation by adding comprehensive UI testing strategy guidelines. The changes provide clear direction on how to test different aspects of UI code using appropriate tools for each testing layer.

Changes:

• Added UI testing strategy section with guidelines for testing Redux actions/selectors, side effects, component rendering, and browser interactions
• Explicitly discourages using @testing-library/react in favor of the documented patterns
• Minor formatting correction (line ending normalization)

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