feat: add PendingUpdateTyped template class

Add PendingUpdateTyped template class to provide type-safe Apply() method
for table metadata changes. This allows different operations to return
specific result types (e.g., vector<Snapshot> for ExpireSnapshots).

Changes:
- Add PendingUpdateTyped<T> template extending PendingUpdate
- Move typed Apply() method from base to template class
- Remove ErrorCollector base from PendingUpdate (simplified design)

This is needed for ExpireSnapshots which returns vector<Snapshot> from Apply().

Author: shangxinli

src/iceberg/pending_update.h

@@ -25,7 +25,6 @@
 #include "iceberg/iceberg_export.h"
 #include "iceberg/result.h"
 #include "iceberg/type_fwd.h"
-#include "iceberg/util/error_collector.h"
 
 namespace iceberg {
 
@@ -37,16 +36,10 @@ namespace iceberg {
 ///
 /// This matches the Java Iceberg pattern where BaseTransaction stores a
 /// List<PendingUpdate> without type parameters.
-class ICEBERG_EXPORT PendingUpdate : public ErrorCollector {
+class ICEBERG_EXPORT PendingUpdate {
  public:
   virtual ~PendingUpdate() = default;
 
-  /// \brief Verify that the changes are valid and apply them.
-  /// \return Status::OK if the changes are valid, or an error:
-  ///         - ValidationFailed: if pending changes cannot be applied
-  ///         - InvalidArgument: if pending changes are conflicting
-  virtual Status Apply() = 0;
-
   /// \brief Apply and commit the pending changes to the table
   ///
   /// Changes are committed by calling the underlying table's commit operation.
@@ -69,4 +62,33 @@ class ICEBERG_EXPORT PendingUpdate : public ErrorCollector {
   PendingUpdate() = default;
 };
 
+/// \brief Template class for type-safe table metadata changes using builder pattern
+///
+/// PendingUpdateTyped extends PendingUpdate with a type-safe Apply() method that
+/// returns the specific result type for each operation. Subclasses implement
+/// specific types of table updates such as schema changes, property updates, or
+/// snapshot-producing operations like appends and deletes.
+///
+/// Apply() can be used to validate and inspect the uncommitted changes before
+/// committing. Commit() applies the changes and commits them to the table.
+///
+/// \tparam T The type of result returned by Apply()
+template <typename T>
+class ICEBERG_EXPORT PendingUpdateTyped : public PendingUpdate {
+ public:
+  ~PendingUpdateTyped() override = default;
+
+  /// \brief Apply the pending changes and return the uncommitted result
+  ///
+  /// This does not result in a permanent update.
+  ///
+  /// \return the uncommitted changes that would be committed, or an error:
+  ///         - ValidationFailed: if pending changes cannot be applied
+  ///         - InvalidArgument: if pending changes are conflicting
+  virtual Result<T> Apply() = 0;
+
+ protected:
+  PendingUpdateTyped() = default;
+};
+
 }  // namespace iceberg