feat: add PendingUpdateTyped template class

Add PendingUpdateTyped template class to provide type-safe ApplyTyped() 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
- Add ApplyTyped() method returning Result<T> for typed operations
- Keep Apply() returning Status in base PendingUpdate for compatibility
- PendingUpdateTyped provides default Apply() implementation that calls ApplyTyped()
- Update ExpireSnapshots to use ApplyTyped() instead of Apply()

This design allows:
- ExpireSnapshots to return Result<vector<Snapshot>> from ApplyTyped()
- UpdateProperties to continue using Status Apply() from base
- All PendingUpdate subclasses to work polymorphically through base interface

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 {
 
@@ -37,7 +36,7 @@ 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;
 
@@ -69,4 +68,44 @@ 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 (typed version)
+  ///
+  /// 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> ApplyTyped() = 0;
+
+  /// \brief Apply the pending changes (base class override)
+  ///
+  /// This implementation calls ApplyTyped() and discards the result.
+  Status Apply() override {
+    auto result = ApplyTyped();
+    if (result.has_error()) {
+      return Status(result.error());
+    }
+    return {};
+  }
+
+ protected:
+  PendingUpdateTyped() = default;
+};
+
 }  // namespace iceberg

src/iceberg/update/expire_snapshots.cc

@@ -54,9 +54,9 @@ ExpireSnapshots& ExpireSnapshots::SetCleanupLevel(CleanupLevel level) {
   return *this;
 }
 
-Result<std::vector<std::shared_ptr<Snapshot>>> ExpireSnapshots::Apply() {
+Result<std::vector<std::shared_ptr<Snapshot>>> ExpireSnapshots::ApplyTyped() {
   // Placeholder implementation - full snapshot expiration logic to be implemented
-  return NotImplemented("ExpireSnapshots::Apply() is not yet implemented");
+  return NotImplemented("ExpireSnapshots::ApplyTyped() is not yet implemented");
 }
 
 Status ExpireSnapshots::Commit() {

src/iceberg/update/expire_snapshots.h

@@ -147,7 +147,7 @@ class ICEBERG_EXPORT ExpireSnapshots
   ///
   /// \return the list of snapshots that would be expired, or an error:
   ///         - ValidationFailed: if pending changes cannot be applied
-  Result<std::vector<std::shared_ptr<Snapshot>>> Apply() override;
+  Result<std::vector<std::shared_ptr<Snapshot>>> ApplyTyped() override;
 
   /// \brief Apply and commit the pending changes to the table
   ///