Fix broken intra-doc links in proto_encode/proto_decode

Module-level doc comments referenced items inside the modules without
qualifying them, so cargo doc with `-D warnings` (CI default) failed
with `unresolved link` warnings. Qualify the links with the
`proto_encode::` / `proto_decode::` module prefix.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: adriangb

.gitignore

@@ -81,3 +81,4 @@ profile.json.gz
 
 # Claude Code personal settings
 .claude/settings.local.json
+.claude/

datafusion/physical-expr-common/src/physical_expr.rs

@@ -478,11 +478,11 @@ pub trait PhysicalExpr: Any + Send + Sync + Display + Debug + DynEq + DynHash {
 
 /// Encode-side context for [`PhysicalExpr::to_proto`].
 ///
-/// Expression authors only ever see [`PhysicalExprEncodeCtx`]: a concrete
-/// struct with stable methods. Internally it dispatches to a
-/// [`PhysicalExprEncode`] implementor that lives in `datafusion-proto`,
-/// which is what lets `physical-expr-common` stay free of `datafusion-proto`
-/// as a dep.
+/// Expression authors only ever see [`proto_encode::PhysicalExprEncodeCtx`]:
+/// a concrete struct with stable methods. Internally it dispatches to a
+/// [`proto_encode::PhysicalExprEncode`] implementor that lives in
+/// `datafusion-proto`, which is what lets `physical-expr-common` stay free
+/// of `datafusion-proto` as a dep.
 ///
 /// More specialized helpers (e.g. encoding UDFs/UDAFs/UDWFs through the
 /// extension codec) can be added to the context as expressions migrate;
@@ -536,8 +536,9 @@ pub mod proto_encode {
 /// Decode-side counterpart to [`proto_encode`].
 ///
 /// Expression authors implement an associated `try_from_proto` on their
-/// concrete type and receive a [`PhysicalExprDecodeCtx`]; the central match
-/// in `datafusion-proto` dispatches `ExprType` variants to those constructors.
+/// concrete type and receive a [`proto_decode::PhysicalExprDecodeCtx`]; the
+/// central match in `datafusion-proto` dispatches `ExprType` variants to
+/// those constructors.
 ///
 /// As with the encode side, the public surface is a struct (not a `&dyn`
 /// trait) so future fields/helpers (registries for third-party expressions,