Fix/postgrest not null narrowing

[DevanshTyagi04]

Improves TypeScript type inference for PostgREST filters by correctly narrowing column types after explicit non-null checks.

What changed?

• Added a specialized overload for not(column, 'is', null) in PostgrestFilterBuilder
• Ensured that when a column is filtered to exclude NULL, the resulting row type marks that column as non-nullable
• Preserved existing behavior for all other not() filter usages

Why was this change needed?

Previously, filtering out NULL values using not(column, 'is', null) did not update the inferred row type, leaving the column typed as nullable.
This caused a mismatch between PostgREST semantics and TypeScript inference.

This change aligns the type system with runtime behavior and improves developer ergonomics.

Closes #1360

📸 Screenshots/Examples

// Before
const { data } = await supabase
  .from('users')
  .select()
  .not('email', 'is', null)

// email: string | null ❌

// After
// email: string ✅

[coveralls]

coverage: 80.997%. remained the same
when pulling 25e4da5 on DevanshTyagi04:fix/postgrest-not-null-narrowing
into 09aa106 on supabase:master.

[github-actions]

This PR has been marked as stale because it has not had activity for 90 days.
It will be closed in 14 days if no further activity occurs.

Please rebase and update if this is still needed.