docs(test-utils): add Vitest setup guide and examples

[chrisgervang]

For #10223

Background

Community feedback requested documentation for using @deck.gl/test-utils with Vitest, including setup instructions and working examples.

Change List

• Add "Using with Vitest" section to the test-utils overview with quick start, Vitest Browser Mode setup, and a minimal vitest.config.ts example
• Add Vitest examples to testLayer and generateLayerTests docs alongside existing tape examples
• Update Usage sections to show the @deck.gl/test-utils/vitest import path

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Note

Low Risk
Documentation-only changes with no runtime or API behavior impact.

Overview
Documents how to run layer lifecycle tests with Vitest using the @deck.gl/test-utils/vitest entry point (built-in vi.spyOn()), in response to community requests (#10223).

The overview gains a Using with Vitest section: install notes, why Vitest Browser Mode + Playwright is required (real WebGL2; JSDOM is not supported), a sample vitest.config.ts with SwiftShader flags, and a minimal ScatterplotLayer test.

testLayer and generateLayerTests pages now lead with Vitest examples (alongside existing tape samples) and their Usage blocks show imports from @deck.gl/test-utils/vitest instead of the default package path.

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[coveralls]

coverage: 83.386% (-0.004%) from 83.39% — chr/update-test-utils-docs-vitest into master