feat(logger): migrate tests to Vitest

[Manvi1203]

Summary

Migrates @firebase/logger from legacy Karma + Mocha + Webpack test runner to a unified Vitest multi-project runner (node and browser with Playwright Chromium headless).

Changes

• Replaced legacy Karma and Mocha runners with unified Vitest runner (vitest.config.mjs)
• Added packages/logger/test/polyfills.ts and packages/logger/test/setup.ts
• Added ambient types packages/logger/src/types/vitest-globals.d.ts
• Added root guard scripts/ensure_playwright.js
• Deleted deprecated packages/logger/karma.conf.js
• Kept vitest dependencies scoped to packages/logger/package.json for isolated migration

CI Impact & Verification

• Test Count: 58 passed across 2 projects (29 Node + 29 Browser Chromium) [no regressions]
• Before (Karma + Mocha): ~7.8s total (Mocha 1.98s + Karma/Webpack ~5.8s)
• After (Vitest Unified): 1.42s total execution time

[changeset-bot]

⚠️ No Changeset found

Latest commit: fb3ed13

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[gemini-code-assist]

Code Review

This pull request migrates the testing framework for the logger package from Karma and Mocha to Vitest, introducing new configuration files, setup scripts, and polyfills for both Node.js and browser environments. The feedback highlights several areas for improvement: removing a global 'process.env': {} definition in vitest.config.mjs that breaks Node.js environment variables, using queueMicrotask instead of setTimeout for a more accurate process.nextTick polyfill, applying optional chaining in getTestTitle to prevent potential runtime errors, and removing redundant global type declarations that are already provided by vitest/globals.