Add data-testid to Play and Stop buttons for E2E testing

[Irfan762]

p5.js version

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What is your operating system?

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Web browser and version

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Actual Behavior

Actual Behavior Play and Stop buttons in the toolbar lack data-testid attributes, making them difficult to target reliably in automated E2E tests. Tests currently must rely on CSS classes (fragile) or ARIA labels (not always present).

This makes it harder to write stable E2E tests for the core sketch execution workflow.

Expected Behavior

Expected behavior: Play and Stop buttons should have semantic test identifiers:

data-testid="play-button"
data-testid="stop-button"

Steps to reproduce

Inspect Play button in toolbar
No data-testid attribute present
E2E tests must use fragile selectors like CSS classes

[Irfan762]

@raclim assign me