fix: improve type safety for additional source builders (#2170)

[mkcodes4]

Fixes

Fixes #2170

Description

This PR refactors get-additional-sources.ts to improve type safety and simplify runtime logic.

Key changes:

• Introduced SelectiveRequired<T, K> utility type to enforce required properties after filtering.
• Updated getAdditionalSourceBuilders to use a generic <T extends MediaType> for proper type narrowing.
• Removed a redundant runtime check in getAdditionalSources since TypeScript now guarantees urlFunc is defined.

Testing Instructions

Run type-check on the frontend:

cd frontend
pnpm run types

Checklist

• My pull request has a descriptive title (not a vague title likeUpdate index.md).
• My pull request targets the default branch of the repository (main) or a parent feature branch.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.
• I ran the DAG documentation generator (ov just catalog/generate-docs for catalog
  PRs) or the media properties generator (ov just catalog/generate-docs media-props
  for the catalog or ov just api/generate-docs for the API) where applicable.

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