fix(python): render field types as SQL strings, not Rust Debug

[jackylee-ch]

PyDataField.field_type() returns Rust Debug output — a column declared
VARCHAR(10) reads back as Varchar(VarCharType { length: 10, nullable: true }).
The in-place TODO(#284 follow-up) names the cause: DataType has no enum-level
Display, so there was nothing to call.

Only 10 of the 23 variants carried a leaf Display. This adds the other nine
scalars and the four constructed types, then the dispatch. The constructed formats
fill Java's FORMAT with the children's own SQL strings (ARRAY<%s>,
MAP<%s, %s>, MULTISET<%s>, ROW<%s>), so a child's NOT NULL sits inside the
brackets and the outer one after them; ROW renders fields like
DataField.asSQLString — escaped name, type, then COMMENT '...'. Every keyword
and format string was checked against the Java class rather than inferred.

The dispatch also retires VectorType::element_sql_name, a private table of seven
primitive names with unreachable!() for the rest, which existed only because this
dispatch did not.

cargo test -p pypaimon_rust cannot link libpython here, so the binding is
compile-checked locally and its behaviour left to integration (python).
The rendering is covered by unit tests in paimon.