Write Comprehensive Tests

[stifskere]

Sub-issue of #3

Objective

Ensure reliability across all components.

Tasks

Stage 1: PostgreSQL CREATE TABLE (priority)

• Unit tests for AST builder, parser, diff, and SQL generator on PostgreSQL CREATE TABLE.
• Integration tests for end-to-end parse → diff → generate on PostgreSQL.

Stage 2: Full MVP / Other Dialects

• Extend tests for other dialects and complex schema cases.
• Add edge case and error handling tests.

Note: Testing Stage 2 is less urgent; focus on Stage 1 first for validation within Flaky.