[jasleenkaur-qed42]

Summary

Backport of #22992 to v6.

Saving the config file while another file's HMR update is still in flight (its hotUpdate / handleHotUpdate hooks are being awaited) crashes the dev server with TypeError: Cannot set properties of undefined (setting 'error').

Root Cause

handleHMRUpdate keys hotMap by the environment instances present at the start of the update. A config-change restart replaces server.environments in place, so after the awaited hook the handler looks up environments that were never added to hotMap, and the catch block writes .error on undefined.

Changes

• Cherry-pick of b1186c3: capture an environment snapshot, bail out once the server has been restarted, and use the captured client environment.
• Adds the regression tests from fix(hmr): preserve environment snapshot during server restart #22992.

Testing

• pnpm exec vitest run packages/vite/src/node/server/__tests__/hmr.spec.ts: fails without the change, passes with it
• pnpm run test-unit, HMR playground e2e, typecheck, lint

Related Issue

Closes #23984

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