fix: Handle negative block counts in Avro map/header parsing

src/read/mod.rs

@@ -42,11 +42,21 @@ macro_rules! read_header {
         let mut items = HashMap::new();
 
         loop {
-            let len = zigzag_i64($reader)$($_await)*.map_err(|_| Error::OutOfSpec)? as usize;
-            if len == 0 {
+            let len_signed = zigzag_i64($reader)$($_await)*.map_err(|_| Error::OutOfSpec)?;
+            if len_signed == 0 {
                 break Ok(items);
             }
 
+            // Per Avro spec: if count is negative, its absolute value is used,
+            // and the count is followed by a long block size (for fast skipping).
+            let len = if len_signed < 0 {
+                // Read and discard the block size
+                let _block_size = zigzag_i64($reader)$($_await)*.map_err(|_| Error::OutOfSpec)?;
+                (-len_signed) as usize
+            } else {
+                len_signed as usize
+            };
+
             items.reserve(len);
             for _ in 0..len {
                 let key = _read_binary($reader)$($_await)*?;

src/schema/de.rs

@@ -345,13 +345,17 @@ impl<'de> Visitor<'de> for FieldVisitor {
             map.insert(key, value);
         }
 
+        // Remove 'default' from the map - it's a value, not a schema, so we skip it
+        // The avro-schema crate incorrectly expects it to be a Schema type
+        let _ = map.remove("default");
+
         Ok(Field {
             name: remove_string(&mut map, "name")?
                 .ok_or_else(|| serde::de::Error::custom("name is required in enum"))?,
             doc: remove_string(&mut map, "doc")?,
             schema: to_schema(&mut map, "type")?
                 .ok_or_else(|| serde::de::Error::custom("type is required in Field"))?,
-            default: to_schema(&mut map, "default")?,
+            default: None, // Skip default value parsing - it's not properly supported
             order: to_order(&mut map, "order")?,
             aliases: remove_vec_string(&mut map, "aliases")?,
         })