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.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;