fix: Address PR review comments for EqualityDeleteWriter

- Add TODO comment for encryption key_metadata field
- Populate partition_spec_id from spec in DataFile metadata
- Add test assertion for partition_spec_id

Author: shangxinli

src/iceberg/data/equality_delete_writer.cc

@@ -23,6 +23,7 @@
 
 #include "iceberg/file_writer.h"
 #include "iceberg/manifest/manifest_entry.h"
+#include "iceberg/partition_spec.h"
 #include "iceberg/util/macros.h"
 
 namespace iceberg {
@@ -101,6 +102,8 @@ class EqualityDeleteWriter::Impl {
         .split_offsets = std::move(split_offsets),
         .equality_ids = options_.equality_field_ids,
         .sort_order_id = options_.sort_order_id,
+        .partition_spec_id =
+            options_.spec ? std::make_optional(options_.spec->spec_id()) : std::nullopt,
     });
 
     FileWriter::WriteResult result;

src/iceberg/data/equality_delete_writer.h

@@ -50,6 +50,7 @@ struct ICEBERG_EXPORT EqualityDeleteWriterOptions {
   std::vector<int32_t> equality_field_ids;
   std::optional<int32_t> sort_order_id;
   std::unordered_map<std::string, std::string> properties;
+  // TODO: add key_metadata for encryption
 };
 
 /// \brief Writer for Iceberg equality delete files.