Exec-server filesystem calls should preserve cross-platform `file:` URIs
across the remote boundary instead of converting them through paths
native to the client host.
This changes the exec-server filesystem protocol DTOs to use `PathUri`,
carries those values directly through remote and sandbox-helper
transports, and keeps legacy native absolute-path request strings
readable for compatibility. It also updates protocol documentation and
coverage for URI serialization and non-native URI forwarding.