fix(drizzle): support filtering polymorphic hasMany relationship fields

[naaa760]

what?

• Support filtering polymorphic hasMany relationship fields in REST and the admin list view.

Why?

• Filtering on polymorphic hasMany fields (e.g. where[authors][exists]=true or polymorphic equals) caused APIError: Not supported because the path resolution logic didn’t handle these cases.

How?

• In getTableColumnFromPath.ts: handle the exists filter by returning the relationship table’s parent column, and support polymorphic equals when the value is an array (e.g. from the admin UI).
• In sanitizeQueryValue.ts: normalize polymorphic values when they come as a single-element array so the equals logic works correctly.

Fixes #15909

[r1tsuu]

Thanks for the PR! This definitely needs an integration test, I'd put this to test/relationships/int.spec.ts

[atimmer]

This fixes the issue when filtering for specific authors and also when filtering for 'has an author' or 'doesn't have an author'.

There is one specific case where it still errors. When I try to filter for authors value "Nothing" I get an error:

## Error Type
Runtime TypeError

## Error Message
Cannot destructure property 'hasMany' of 'n' as it is undefined.

Next.js version: 15.5.9 (Webpack)

Basically this URL: http://localhost:3105/admin/collections/articles?depth=1&limit=10&where%5Bor%5D%5B0%5D%5Band%5D%5B0%5D%5Bauthors%5D%5Bequals%5D%5B0%5D%5Bvalue%5D=null&page=1