fix(build): use resolved base for chunk import map keys

[lazerg]

When build.chunkImportMap is enabled, the baseUrl handed to rolldown is always the resolved base, overwriting anything set in rolldownOptions.experimental.chunkImportMap.baseUrl. getImportMapBaseUrl still read that user value back instead, which is either unset or already overwritten, so it fell back to / whenever base wasn't /. That mismatch made the import map keys and the mapping lookup diverge, dropping __vite__mapDeps and modulepreload/CSS references for lazy chunks.

getImportMapBaseUrl is only ever reached behind that same chunkImportMap flag, so it can just return the resolved base directly.

fixes #23350

[github-actions]

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