[Pg-kit]: Fix enum migration when dropping values used as defaults

[jrhe]

Summary

Fixes PostgreSQL enum migration errors when dropping enum values that are used as column defaults.

Problem

When attempting to drop an enum value that is currently used as a default value for a column, PostgreSQL throws an error:

cannot drop type "enum_name" because other objects depend on it

This occurs because PostgreSQL cannot drop or modify an enum type while it's still referenced by a default constraint.

Solution

Added logic to drop default constraints before converting columns to text type during enum modifications. This ensures proper handling of dependencies when enum values are dropped.

Changes

• Modified AlterTypeDropValueConvertor in sqlgenerator.ts to add DROP DEFAULT statements
• Updated all affected tests to expect the new DROP DEFAULT statements
• Added specific test case for the reported issue

Test plan

• All existing enum tests pass with updated expectations
• New test case specifically validates the fix for dropping enum values used as defaults
• Migration generation correctly handles enum modifications with default values

Fixes #4295

🤖 Generated with Claude Code

[jrhe]

The singlestore tests failed as I do not have an API key. I haven't touched any singlestore related files so I think it should be ok, but would appreciate it if someone could double check.

PR co-authored with claude code but I ended up re-writing from scratch.

A large number of tests have been changed as this adds the DROP DEFAULT to any type that is being altered/dropped. From my testing, this should not cause issues when there is no default specified, and just do nothing. We would ideally only do it for those which had a default previously, but statement object does not include details of the previously default set on the enum, only the new one. I have looked into adding this to the diffing engine but I am not familiar enough with it to do so.