Fix primary key detection for column-level constraints (go-zero issue #4967)

[aburizalpurnama]

Description

This PR fixes an issue where model generation fails with the error "missing primary key" when the primary key is defined using column-level constraints instead of table-level constraints.

Problem

Currently, ddl-parser only detects primary keys defined at the table level, which causes it to miss valid primary key definitions that are declared at the column level.

Solution

• Updated the primary key detection logic to also include column-level primary key definitions.
• Ensures compatibility with more SQL schema styles.

Testing

• Added unit tests to verify both table-level and column-level primary key detection.
• Confirmed model generation now works correctly with column-level constraints.

Related Issue

Fixes #9