fix(jdk-codemodel): replace raw String operator fields with typed enums

[deer]

BitwiseBinary, PrefixUnary, PostfixUnary, and CompoundAssignment stored their operator as t.getKind().toString() — an unconstrained string that consumers had to parse to do anything meaningful with. This is the same class of problem as the string type names fixed previously.

Introduces four enums (BitwiseOperator, PrefixOperator, PostfixOperator, AssignmentOperator) covering the exact set of operators each node can hold. The converter switch expressions now map Tree.Kind directly to enum constants, making the mapping explicit and exhaustive. Marshalling preserves the wire format as String (via name()/valueOf()). Tests cover one representative operator per node type.