refactor: add try_to_proto to HashTableLookupExpr

Part of #22435. Adds try_to_proto to HashTableLookupExpr so it
participates in the expression-local serialization pattern introduced
in #21929.

HashTableLookupExpr holds a runtime Arc<Map> that cannot be serialized,
so try_to_proto replaces it with lit(true). This is safe because the
filter is a performance optimisation only — lit(true) passes all rows
and the join produces correct results either way.

The centralized arm in to_proto.rs remains as a fallback for now.

Author: Anurag Tryambak Raut

datafusion/physical-plan/src/joins/hash_join/partitioned_hash_eval.rs

@@ -242,6 +242,8 @@ impl HashTableLookupExpr {
     }
 }
 
+
+
 impl std::fmt::Debug for HashTableLookupExpr {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         let cols = self
@@ -289,6 +291,38 @@ impl PartialEq for HashTableLookupExpr {
 
 impl Eq for HashTableLookupExpr {}
 
+#[cfg(feature = "proto")]
+impl HashTableLookupExpr {
+    /// Serialize this expression to protobuf.
+    ///
+    /// `HashTableLookupExpr` holds an `Arc<Map>` (a runtime hash table built
+    /// on the build side) which cannot be serialized. We replace it with
+    /// `lit(true)`, which is safe because:
+    ///
+    /// - The filter is a performance optimisation, not a correctness requirement.
+    /// - `lit(true)` passes all rows so no valid rows are lost.
+    /// - In distributed execution the remote worker has no access to the
+    ///   build-side hash table anyway.
+    pub fn try_to_proto(
+        &self,
+        _ctx: &datafusion_physical_expr_common::physical_expr::proto_encode::PhysicalExprEncodeCtx<'_>,
+    ) -> datafusion_common::Result<Option<datafusion_proto_models::protobuf::PhysicalExprNode>>
+    {
+        use datafusion_proto_common::scalar_value::Value;
+        use datafusion_proto_common::ScalarValue;
+        use datafusion_proto_models::protobuf;
+        use datafusion_proto_models::protobuf::physical_expr_node::ExprType;
+
+        let value = ScalarValue {
+            value: Some(Value::BoolValue(true)),
+        };
+        Ok(Some(protobuf::PhysicalExprNode {
+            expr_id: None,
+            expr_type: Some(ExprType::Literal(value)),
+        }))
+    }
+}
+
 impl Display for HashTableLookupExpr {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(f, "{}", self.description)