fix: StructType fails to deserialize JSON with type field

[mbutrovich]

This fix enables proper interoperability with Iceberg implementations that include the type discriminator field in their JSON serialization (such as Apache Iceberg Java).

Which issue does this PR close?

Partially address #1749.

What changes are included in this PR?

Fixes a bug in the StructType deserializer that causes JSON deserialization to fail with the error expected ',' or '}' at line 1 column 8.

Root Cause:
The StructType::Deserialize implementation was not properly consuming the "type" field value when deserializing JSON like:

{"type":"struct","fields":[{"id":1,"name":"foo","required":true,"type":"string"}]}

In serde's visitor pattern, when you call map.next_key(), you must call map.next_value() to consume the corresponding value, even if you're discarding it. The deserializer was calling next_key() for the "type" field but not consuming its value, causing the parser to remain stuck at the value position and fail when encountering the next field.

Changes:

• Modified StructTypeVisitor::visit_map to properly consume the type field value
• Added validation that the type field equals "struct"

Are these changes tested?

Yes, two new unit tests have been added to crates/iceberg/src/spec/datatypes.rs:

1. struct_type_with_type_field - Verifies that StructType successfully deserializes from JSON containing the "type":"struct" field. This test would fail before the fix with the error expected ',' or '}' at line 1 column 8

2. struct_type_rejects_wrong_type - Validates that the deserializer properly rejects JSON with incorrect type values (e.g., "type":"list")

[liurenjie1024]

Thaks @mbutrovich for this fix!