fix: keep the Vitest test config when solid-js is already external

[RaphaelFakhri]

Summary

In test mode the plugin only returns its test config when it also sets test.server. When the user config already lists solid-js in test.server.deps.external, the plugin skips test.server and drops the whole test block with it, so Vitest gets neither the jsdom environment nor the @testing-library/jest-dom setup file. Component tests that rely on the DOM or on jest-dom matchers then fail.

This started with #183, which moved the config into a fresh test object but kept test.server as the condition for returning it.

Changes

• Return test whenever it has any key, the same check the next branch uses.
• Add a test:unit script, a root vitest.config.ts and src/test-config.test.ts, and run the unit tests in the Vitest workflow.
• Add a patch changeset.

Testing

pnpm run test:unit: 2 passed. With the src/index.ts change reverted, the new external solid-js case fails (expected undefined to be 'jsdom'). pnpm run build and Prettier pass.

Fixes #454

[changeset-bot]

🦋 Changeset detected

Latest commit: d0e47f1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
vite-plugin-solid	Patch

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