refactor: change NewExpireSnapshots to return unique_ptr

Change Table::NewExpireSnapshots() and Transaction::NewExpireSnapshots()
to return std::unique_ptr instead of std::shared_ptr for consistency
with other factory methods (NewScan, NewTransaction) that create new
objects with exclusive ownership transfer to the caller.

Author: shangxinli

src/iceberg/table.cc

@@ -115,8 +115,8 @@ std::unique_ptr<Transaction> Table::NewTransaction() const {
   throw NotImplemented("Table::NewTransaction is not implemented");
 }
 
-std::shared_ptr<iceberg::ExpireSnapshots> Table::NewExpireSnapshots() {
-  return std::make_shared<iceberg::ExpireSnapshots>(this);
+std::unique_ptr<iceberg::ExpireSnapshots> Table::NewExpireSnapshots() {
+  return std::make_unique<iceberg::ExpireSnapshots>(this);
 }
 
 const std::shared_ptr<FileIO>& Table::io() const { return io_; }

src/iceberg/table.h

@@ -117,8 +117,8 @@ class ICEBERG_EXPORT Table {
 
   /// \brief Create a new expire snapshots operation for this table
   ///
-  /// \return a shared pointer to the new ExpireSnapshots operation
-  virtual std::shared_ptr<ExpireSnapshots> NewExpireSnapshots();
+  /// \return a unique pointer to the new ExpireSnapshots operation
+  virtual std::unique_ptr<ExpireSnapshots> NewExpireSnapshots();
 
   /// \brief Returns a FileIO to read and write table data and metadata files
   const std::shared_ptr<FileIO>& io() const;

src/iceberg/transaction.h

@@ -46,8 +46,8 @@ class ICEBERG_EXPORT Transaction {
 
   /// \brief Create a new expire snapshots operation for this transaction
   ///
-  /// \return a shared pointer to the new ExpireSnapshots operation
-  virtual std::shared_ptr<ExpireSnapshots> NewExpireSnapshots() = 0;
+  /// \return a unique pointer to the new ExpireSnapshots operation
+  virtual std::unique_ptr<ExpireSnapshots> NewExpireSnapshots() = 0;
 
   /// \brief Apply the pending changes from all actions and commit
   ///