Coins: Add kHeader to CDBBatch::size_estimate

The initialization of the manual `size_estimate` in `CDBBatch::Clear()` is corrected from `0` to `kHeader` (LevelDB's fixed batch header size).
This aligns the manual estimate with LevelDB's actual size immediately after clearing, fixing discrepancies that would otherwise be caught by tests in the next commit (e.g., `coins_tests`, `validation_chainstatemanager_tests`).

src/dbwrapper.cpp

@@ -158,14 +158,18 @@ struct CDBBatch::WriteBatchImpl {
 
 CDBBatch::CDBBatch(const CDBWrapper& _parent)
     : parent{_parent},
-      m_impl_batch{std::make_unique<CDBBatch::WriteBatchImpl>()} {};
+      m_impl_batch{std::make_unique<CDBBatch::WriteBatchImpl>()}
+{
+    Clear();
+};
 
 CDBBatch::~CDBBatch() = default;
 
 void CDBBatch::Clear()
 {
     m_impl_batch->batch.Clear();
-    size_estimate = 0;
+    assert(m_impl_batch->batch.ApproximateSize() == kHeader);
+    size_estimate = kHeader; // TODO remove
 }
 
 void CDBBatch::WriteImpl(std::span<const std::byte> key, DataStream& ssValue)

src/dbwrapper.h

@@ -75,6 +75,8 @@ class CDBBatch
     friend class CDBWrapper;
 
 private:
+    static constexpr size_t kHeader{12}; // See: src/leveldb/db/write_batch.cc#L27
+
     const CDBWrapper &parent;
 
     struct WriteBatchImpl;