Switch testing from mocha/chai/karma/c8 to vitest.

[davidlehn]

Karma is unmaintained. Vitest replaces the runner, the browser test harness, and the coverage tool with a single dependency and config.

• Add vitest.config.js with node and browser projects. The browser project runs Chromium through @vitest/browser-playwright, replacing karma and karma-webpack.
• Keep the existing should-style assertions unchanged. tests/setup.js installs the global should from vitest's re-exported chai, so chai is no longer a dependency.
• Start the HTTP/HTTPS test servers in tests/globalSetup.js and pass their ephemeral hosts to tests with inject(). This replaces starting them in the karma config and injecting the hosts via webpack's DefinePlugin, and lets tests/utils-browser.js be removed.
• Split the suite by environment into 10-client-api.spec.js (shared), 20-node.spec.js, and 30-browser.spec.js instead of branching on isNode at runtime. This keeps node-only modules out of the browser project and drops the detect-node dependency. Test bodies are unchanged; both projects still run 17 tests.
• Report coverage with @vitest/coverage-v8 across both projects, so browser-only code paths are now covered. Reported totals shift slightly because vitest and c8 count executable lines differently.
• Replace the test-karma CI job with test-browser, and install and cache Playwright's Chromium in the browser and coverage jobs.

Add an exports field with a browser condition for agentCompatibility and import it via a self-reference. Vite does not apply the top-level browser field to package-internal relative imports, so this is also a fix for browser bundlers, which would otherwise pull undici into their builds.