ASTcolumn_type.toString() doesn't handle UUID type

[matsnow]

Description

Schemas containing UUID columns cause an IllegalArgumentException at runtime when generating diff output.

Steps to Reproduce

1. Create an empty DDL file and a DDL file with a UUID column:

   -- empty.sql
   -- (empty)
   
   -- new.sql
   CREATE TABLE test (id UUID NOT NULL) PRIMARY KEY (id);

2. Run:

   java -jar spanner-ddl-diff.jar --originalDdlFile empty.sql --newDdlFile new.sql --outputDdlFile diff.sql
   

3. Exception thrown:

   Exception in thread "main" java.lang.IllegalArgumentException: Unknown column type UUID
        at com.google.cloud.solutions.spannerddl.parser.ASTcolumn_type.toString(ASTcolumn_type.java:75)
   

Root Cause

ASTcolumn_type.java:toString() has a switch statement that handles almost all types such as FLOAT32, INT64, BOOL, etc — but not UUID.

The parser correctly recognizes UUID in ddl_parser.jjt (line 393), so parsing succeeds, but the Java code that converts the AST back to DDL text fails at the switch statement's default branch.

Suggested Fix

Add case "UUID": to the switch in ASTcolumn_type.toString(), falling through to return typeName; (same pattern as other parameterless types like BOOL, DATE).