refactor(proto): keep the datasource proto module private

The module holds nothing but `TryFrom` impls, and trait impls are
registered globally by coherence, so they reach every downstream crate
whether or not the module that writes them is reachable by path. `pub`
therefore adds no capability: it only publishes an item-less module page
and a path we would then owe compatibility to.

If a later conversion in this family needs a named helper that `TryFrom`
cannot express (an encode/decode context, per #23494), exporting the
module again is additive.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: adriangb

datafusion/datasource/src/mod.rs

@@ -44,7 +44,7 @@ pub mod projection;
 /// Protobuf conversions for [`FileRange`], [`PartitionedFile`] and
 /// [`FileGroup`](crate::file_groups::FileGroup), gated on the `proto` feature.
 #[cfg(feature = "proto")]
-pub mod proto;
+mod proto;
 pub mod schema_adapter;
 pub mod sink;
 pub mod source;