fix(optimizer): avoid false-mapping warnings (fix #23512)

[QIU-Guanzong]

Fixes #23512.

An explicit browser mapping to false resolves to Vite's bare browserExternalId. The dependency optimizer treated it like a regular browser-externalized builtin and installed the development Proxy, which emitted warnings despite the browser-field behavior being an empty module.

This routes the bare sentinel to an empty CJS module in both development and production. Sentinel IDs with a suffix continue through the existing Proxy path, so unsupported Node builtins retain their diagnostics.

Tests:

• npx --yes pnpm@12.4.2 --filter=./packages/vite run build
• npx --yes pnpm@12.4.2 run test-serve playground/optimize-deps/__tests__/optimize-deps.spec.ts
• npx --yes pnpm@12.4.2 run test-build playground/optimize-deps/__tests__/optimize-deps.spec.ts
• npx --yes pnpm@12.4.2 run test-serve playground/resolve/__tests__/resolve.spec.ts
• targeted ESLint, Oxfmt, and git diff --check

AI assistance disclosure: AI assistance was used to investigate, prepare, and validate this focused change.

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