fix(coverage): correct coverage when isolate: false is used

[AriPerkkio]

Description

• Fixes Wrong coverage when using isolate: false #6736
• Istanbul: Preserve files in coverage state, reset just the counters
• V8: Don't restart coverage collection when same runtime instance is re-used between tests

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[pkg-pr-new]

@vitest/browser

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commit: c89f42b

[AriPerkkio]

/ecosystem-ci run

(interested in vitest-coverage-large and aria-live-capture)

[vitest-ecosystem-ci]

📝 Ran ecosystem CI: Open

suite	result
aria-live-capture	✅ success
nuxt	❌ failure
nuxt-test-utils	❌ failure
elk	✅ success
effect	✅ success
lerna-lite	✅ success
zustand	✅ success
vue	✅ success
vite	✅ success
vitest-vscode	❌ failure
vitest-sonar-reporter	❌ failure
vitest-github-actions-reporter	❌ failure
vitest-browser-simple	✅ success
vitest-browser-examples	❌ failure
vitest-coverage-large	✅ success
vitest-reporters-large	✅ success
vitest-benchmark-large	✅ success