Fix Firefox test build: add import.meta.env?.DEV replacement plugin to vite.config.mjs

[Copilot]

Firefox 115 ESR tests were failing with TypeError: COMPUTED_GETTERS is undefined because Vite does not natively replace import.meta.env?.DEV (optional chaining) during builds — only import.meta.env.DEV (without ?.) is handled by Vite's built-in env replacement. When import.meta.env?.DEV goes unreplaced in the build output, it evaluates to undefined at runtime in the browser (since import.meta.env is a Vite build-time artifact, not present at runtime), leaving COMPUTED_GETTERS uninitialized.

The rollup.config.mjs already had a define custom import.meta.env plugin that replaces import.meta.env?.DEV with true/false, but vite.config.mjs (used for the Firefox CI test build via pnpm vite build --mode=development) was missing it.

Changes

• vite.config.mjs: Added a define custom import.meta.env Vite plugin (placed before the Babel plugin) that replaces import.meta.env?.DEV with true in development mode and false in production mode — mirroring the equivalent plugin already present in rollup.config.mjs.

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[NullVoxPopuli]

@copilot, this fix is not appropriate, given the original change here: https://github.com/emberjs/ember.js/pull/21115/changes#diff-6915e9ed2703a5079ad681e814d82b43a93abc150305760d2f640cff21de2bd2

It implies that we build the project wrong, and import.meta.env?.DEV is false.

When we build for the firefox tests, import.meta.env?.DEV should be true

[Copilot]

@copilot, this fix is not appropriate, given the original change here: https://github.com/emberjs/ember.js/pull/21115/changes#diff-6915e9ed2703a5079ad681e814d82b43a93abc150305760d2f640cff21de2bd2

I...

You're right. Reverted the decorator.ts change.

The real issue is that Vite doesn't natively replace import.meta.env?.DEV (optional chaining) — only import.meta.env.DEV (without ?.). The rollup.config.mjs already had a define custom import.meta.env plugin that does this replacement, but vite.config.mjs (used for the Firefox test build via pnpm vite build --mode=development) was missing it.

Fixed in 462a664 by adding the same plugin to vite.config.mjs, placed before the Babel plugin to ensure the pattern is replaced before Babel could transform it.