fix(testing): use isBrowser check in leftover files

[saboooor]

What is it?

• Bug

Description

adding on to #8584 fixing leftover code :)

Checklist

• My code follows the developer guidelines of this project
• I performed a self-review of my own code

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Latest commit: 5947965

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[Copilot]

Pull request overview

This PR continues the testing-related cleanup from #8584 by removing ad-hoc import.meta.env.TEST environment detection and standardizing on isBrowser from @qwik.dev/core/build in remaining internal modules.

Changes:

• Switch scheduleEffects() to use imported isBrowser instead of a test-only isServerPlatform()/isServer fallback.
• Switch preloader bundle-graph to use imported isBrowser and remove isServerPlatform/isServer imports.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/qwik/src/core/reactive-primitives/utils.ts	Uses isBrowser from @qwik.dev/core/build to gate client-only vnode diff / node-prop payload behavior.
packages/qwik/src/core/preloader/bundle-graph.ts	Uses isBrowser from @qwik.dev/core/build for browser-only preloader initialization and removes test-only environment branching.

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