[compiler-segfault]

Which issue does this PR close?

• Closes Non-finite defaults cause panics during serde #3627.

What changes are included in this PR?

NestedField is serialized through SerdeNestedField, and the From<NestedField> conversion used expect when a default value could not be converted to JSON. A field whose default does not fit its type therefore panicked inside serde_json::to_string, which is reachable from with_initial_default / with_write_default:

let field = NestedField::new(1, "f", Type::Primitive(PrimitiveType::Float), false)
    .with_initial_default(Literal::string("1e39"));
serde_json::to_string(&field); // panics

The conversion is now fallible (TryFrom<NestedField> for SerdeNestedField), and NestedField's hand-written Serialize passes that error through serde::ser::Error::custom, so serialization fails instead of aborting the process. Nothing else about the JSON shape changes.

Are these changes tested?

Two tests in spec::datatypes::tests cover the initial and the write default: both panic before the change and assert the serialization error afterwards. cargo test -p iceberg --lib passes (2141 passed, 1 ignored), and cargo clippy -p iceberg --all-targets -- -D warnings and cargo fmt are clean.

AI Disclosure

The change and this description were prepared with an AI coding agent (DeepSeek Harness, deepseek-flash). The diff and the test output were reviewed before submitting.

Uncertainty worth a reviewer's attention: whether reporting the conversion failure through serde::ser::Error::custom is the right surface for it, or whether Serialize should stay infallible and the type/validity check should move into with_initial_default / with_write_default instead.

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