fix: add missing @IcebergApi annotations

Add annotations to:
- AbstractColumnReader.nativeHandle (protected field accessed by Iceberg
  subclasses)
- AbstractCometSchemaImporter.close() (called by Iceberg)

Also update documentation to include these APIs.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: andygrove

common/src/main/java/org/apache/arrow/c/AbstractCometSchemaImporter.java

@@ -19,6 +19,8 @@
 
 package org.apache.arrow.c;
 
+import org.apache.comet.IcebergApi;
+
 import org.apache.arrow.memory.BufferAllocator;
 import org.apache.arrow.vector.FieldVector;
 import org.apache.arrow.vector.types.pojo.Field;
@@ -67,6 +69,7 @@ public FieldVector importVector(ArrowArray array, ArrowSchema schema) {
     return vector;
   }
 
+  @IcebergApi
   public void close() {
     provider.close();
   }

common/src/main/java/org/apache/comet/parquet/AbstractColumnReader.java

@@ -64,6 +64,7 @@ public abstract class AbstractColumnReader implements AutoCloseable {
   protected int batchSize;
 
   /** A pointer to the native implementation of ColumnReader. */
+  @IcebergApi
   protected long nativeHandle;
 
   AbstractColumnReader(

docs/source/contributor-guide/iceberg_public_api.md

@@ -131,6 +131,9 @@ public Map<String, String> getLogicalTypeParams()
 Base class for column readers.
 
 ```java
+// Protected field accessed by Iceberg subclasses
+protected long nativeHandle
+
 // Methods used by Iceberg
 public void setBatchSize(int batchSize)
 public void close()
@@ -240,6 +243,17 @@ Imports and converts schemas between Arrow and Spark formats.
 // Constructor
 public CometSchemaImporter(RootAllocator allocator)
 
+// Methods used by Iceberg (inherited from AbstractCometSchemaImporter)
+public void close()
+```
+
+## Package: `org.apache.arrow.c`
+
+### AbstractCometSchemaImporter
+
+Base class for `CometSchemaImporter`.
+
+```java
 // Methods used by Iceberg
 public void close()
 ```