decimal type serialized as decimal(P,S) — diverges from spec/java/python

[SreeramGarlapati]

problem

iceberg-rust serializes the decimal type as decimal(9,2) (no space after comma). both java and python serialize as decimal(9, 2) (with space). the iceberg spec documents the type as decimal(P, S).

strict downstream parsers that expect the canonical form reject the spaceless variant.

current behavior

// crates/iceberg/src/spec/datatypes.rs
write!(f, "decimal({precision},{scale})")

produces: "decimal(9,2)"

expected behavior

write!(f, "decimal({precision}, {scale})")

produces: "decimal(9, 2)" — matching java (String.format("decimal(%d, %d)", ...)) and python (f"decimal({self.precision}, {self.scale})").

impact

• all existing parsers (java, python, rust) handle both forms via .trim() — no breaking change
• aligns iceberg-rust output with the canonical form used by the other two major implementations
• one-character fix in the format string

notes

the deserializer already handles both formats correctly (trims whitespace after comma split). this is purely about output alignment.