fix: don't assume a test section when vite runs in test mode

[DylanPiercey]

In test mode the plugin destructured config.test and read test.environment without guarding it. vitest resolves a plain vite.config.ts — which has no test section — so merely having @marko/vite installed made vitest run fail before a single test loaded:

TypeError: Cannot read properties of undefined (reading 'environment')
    at BasicMinimalPluginContext.config (@marko/vite/dist/index.mjs)

That pushes consumers into maintaining a separate vitest.config.ts purely to keep the site plugins out of the way, which is what surfaced this on markojs.com.

The dom-environment branch is unchanged when a test section is present; test.execArgv is only touched inside that branch, where test is known to exist.

Verified against the real failure: with a locally built dist swapped into markojs.com's node_modules, vitest run --config vite.config.ts goes from the TypeError above to Test Files 1 passed / Tests 6 passed.

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🦋 Changeset detected

Latest commit: 038e45d

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

This PR includes changesets to release 1 package

Name	Type
@marko/vite	Patch

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