fix(build): preserve preload deps in chunkImportMap when base is not / (fix #23350)

[santusht06]

Description

Fixes #23350.

When build.chunkImportMap is enabled and base is configured to a non-root path (e.g. base: '/sub/'):

1. Rolldown prefixes the chunk import map keys with baseUrl: base (e.g. /sub/assets/...).
2. When reading the map back in getImportMapBaseUrl(), it checked options.build.rolldownOptions.experimental?.chunkImportMap?.baseUrl and defaulted to '/'.
3. Stripping length 1 left keys as 'sub/assets/...', which mismatched bundle keys ('assets/...').
4. As a result, the lookup in importAnalysisBuild.ts missed, dropping the __vite__mapDeps dependency list from lazy chunks and leaving lazy CSS un-preloaded.

Solution

• Update getImportMapBaseUrl in packages/vite/src/node/plugins/html.ts to return (options as any).base ?? '/' when chunkImportMap.baseUrl is not explicitly overridden on rolldownOptions.
• Added unit test in packages/vite/src/node/__tests__/build.spec.ts ensuring preload dependency mappings are preserved with non-root base.

Validation

• pnpm run build
• pnpm vitest run packages/vite/src/node/__tests__/build.spec.ts (68/68 passed)
• pnpm run typecheck (0 errors)
• pnpm run lint (0 errors)
• Verified with minimal reproduction repository.

[official-burak]

The fallback is the right idea. resolveRolldownOptions injects baseUrl: base only into the object it passes to Rolldown, so environment.config.build.rolldownOptions.experimental.chunkImportMap never gets it unless the user set it themselves. Reading chunkImportMap.baseUrl from that config will keep missing, which is why the strip falls back to '/' and the preload map drops.

ResolvedEnvironmentOptions already has base?: string, so this does not need as any:

return options.base ?? '/'

A second fixture with base: './' would be useful. That is the other value Vite uses, and k.slice(baseUrl.length) depends on the exact prefix Rolldown wrote.

[santusht06]

Thanks for the review @official-burak!

Updated in latest commit:

• Removed as any by typing options: ResolvedEnvironmentOptions & { base?: string }.
• Added test case for base: './' (relative base).