feat(core): preserve env presets instead of inlining

[elecmonkey]

Summary

Stop inlining Rsbuild's default env presets so downstream consumers can replace them:

• process.env.BASE_URL / process.env.ASSET_PREFIX: preserved for all formats.
• import.meta.env.*: preserved for ESM only.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[elecmonkey]

@codex review

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Retain UMD preset values before stubbing process.env

When a UMD source reads process.env.BASE_URL or process.env.ASSET_PREFIX without a user override, those keys have already been removed from nextDefine, and this parent definition then compiles the access against an empty object, yielding undefined instead of Rsbuild's previous "/"/"" defaults. The new UMD test only detects the unrelated "production" literal, so it misses this runtime value regression; preserve these preset literals for UMD before stubbing unknown process.env properties.

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