Right now, the IndexBuildsCoordinator receives a collation in the storage format, where the absence of a collation is interpreted as a simple collation. This is very error-prone, because in other parts of the system the absence of a collation can instead mean "use the collection's default collation."

The goal of this ticket is to ensure the collation is always passed explicitly up to the IndexBuildsCoordinator API. The coordinator will then be responsible for converting the specified collation into the storage format.

This change is important because it removes an ambiguous and fragile convention from a critical path in index creation. By making the collation explicit, we reduce the risk of subtle bugs, ensure consistent behavior across components, and make the code easier to reason about and maintain over time.