fix(core-flows): avoid overriding customer and region from setPricingContext hook result (#14022)

## Summary

Updated context passed for each variant when calculating prices so that `region` and `customer` coming from `setPricingContext` are not overriden if provided.

*Please provide answer here*

Otherwise you can't provide your own `region` and `customer` objects with additional information and you are left with the ones we pass as part of the cart object to the workflows that execute the hook.

*Please provide answer here*

Changed the order in which we define the default `region` and `cart` fields, taking their value from the cart, to before the destructured `setPricingContextResult`.

*Please provide answer here*

**Testing** — How have these changes been tested, or how can the reviewer test the feature?

*Please provide answer here*

---

## Examples

Provide examples or code snippets that demonstrate how this feature works, or how it can be used in practice.  
This helps with documentation and ensures maintainers can quickly understand and verify the change.

```ts
// Example usage
```

---

## Checklist

Please ensure the following before requesting a review:

- [x] I have added a **changeset** for this PR
    - Every non-breaking change should be marked as a **patch**
    - To add a changeset, run `yarn changeset` and follow the prompts
- [ ] The changes are covered by relevant **tests**
- [x] I have verified the code works as intended locally
- [x] I have linked the related issue(s) if applicable

---

## Additional Context

fixes #13990 
closes SUP-2647


---

> [!NOTE]
> Adjust pricing context merge order so `setPricingContext`-provided `customer` and `region` are not overridden when calculating variant prices.
> 
> - **Core Flows (pricing)**:
>   - In `packages/core/core-flows/src/cart/workflows/get-variants-and-items-with-prices.ts`, change merge order when building `baseContext` so `customer` and `region` from `setPricingContextResult` take precedence over cart-derived values when pricing variants.
> - **Changeset**:
>   - Add patch changeset for `@medusajs/core-flows`.
> 
> <sup>Written by [Cursor Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit 010da73. This will update automatically on new commits. Configure [here](https://cursor.com/dashboard?tab=bugbot).</sup>

Author: NicolasGorga

.changeset/smart-stamps-shake.md

@@ -0,0 +1,5 @@
+---
+"@medusajs/core-flows": patch
+---
+
+fix(core-flows): avoid overriding customer and region from setPricingContext hook result

packages/core/core-flows/src/cart/workflows/get-variants-and-items-with-prices.ts

@@ -105,12 +105,12 @@ export const getVariantsAndItemsWithPrices = createWorkflow(
         const cart = data.cart
         const baseContext = {
           ...filterObjectByKeys(cart, cartFieldsForPricingContext),
+          customer: cart.customer,
+          region: cart.region,
           ...(data.setPricingContextResult ? data.setPricingContextResult : {}),
           currency_code: cart.currency_code ?? cart.region?.currency_code,
           region_id: cart.region_id,
-          region: cart.region,
           customer_id: cart.customer_id,
-          customer: cart.customer,
         }
 
         return (data.items ?? cart.items ?? [])