blockstorage: simplify partial block read validation

Use `SaturatingAdd` following https://github.com/bitcoin/bitcoin/pull/33657#discussion_r2610832092.

src/node/blockstorage.cpp

@@ -1083,7 +1083,7 @@ BlockManager::ReadRawBlockResult BlockManager::ReadRawBlock(const FlatFilePos& p
 
         if (block_part) {
             const auto [offset, size]{*block_part};
-            if (size == 0 || offset >= blk_size || size > blk_size - offset) {
+            if (size == 0 || SaturatingAdd(offset, size) > blk_size) {
                 return util::Unexpected{ReadRawError::BadPartRange}; // Avoid logging - offset/size come from untrusted REST input
             }
             filein.seek(offset, SEEK_CUR);