Refactor `IFileResolver` to reduce dependencies on the file system

The Bicep provider service should not read the file system. We need a better abstraction of `IFileResolver` to reduce dependencies on the file system.

Locate IFileResolver and the Bicep provider service, then trace where the service reads the file system. Define the abstraction boundary and verify that the provider service no longer depends directly on file-system access; the issue provides no named tests or files for validation.

Written by the indexing model from the issue text.