feat(tax): normalize country and province code (#6513)

**What**
- Ensures that country and province codes are always stored as lower case.
- Ensures that calculation context normalizes input before getting region rates.

packages/types/src/tax/mutations.ts

@@ -15,7 +15,7 @@ export interface CreateTaxRegionDTO {
   parent_id?: string | null
   metadata?: Record<string, unknown>
   created_by?: string
-  default_tax_rate: {
+  default_tax_rate?: {
     rate?: number | null
     code?: string | null
     name: string