feat: Region PaymentProvider link (#6577)

**What**

- Introduce link between Region and PaymentProvider
- Introduce API endpoint `GET /store/regions/:id/payment-providers` for retrieving providers by region
- Add tests for both

integration-tests/modules/__tests__/payment/payments.spec.ts

@@ -0,0 +1,74 @@
+import { ModuleRegistrationName, Modules } from "@medusajs/modules-sdk"
+import { IRegionModuleService } from "@medusajs/types"
+import path from "path"
+import { startBootstrapApp } from "../../../environment-helpers/bootstrap-app"
+import { useApi } from "../../../environment-helpers/use-api"
+import { getContainer } from "../../../environment-helpers/use-container"
+import { initDb, useDb } from "../../../environment-helpers/use-db"
+
+jest.setTimeout(50000)
+
+const env = { MEDUSA_FF_MEDUSA_V2: true }
+
+describe("Payments", () => {
+  let dbConnection
+  let appContainer
+  let shutdownServer
+  let regionService: IRegionModuleService
+  let remoteLink
+
+  beforeAll(async () => {
+    const cwd = path.resolve(path.join(__dirname, "..", ".."))
+    dbConnection = await initDb({ cwd, env } as any)
+    shutdownServer = await startBootstrapApp({ cwd, env })
+    appContainer = getContainer()
+    regionService = appContainer.resolve(ModuleRegistrationName.REGION)
+    remoteLink = appContainer.resolve("remoteLink")
+  })
+
+  afterAll(async () => {
+    const db = useDb()
+    await db.shutdown()
+    await shutdownServer()
+  })
+
+  afterEach(async () => {
+    const db = useDb()
+    await db.teardown()
+  })
+
+  it("should list payment providers", async () => {
+    const region = await regionService.create({
+      name: "Test Region",
+      currency_code: "usd",
+    })
+
+    const api = useApi() as any
+    let response = await api.get(
+      `/store/regions/${region.id}/payment-providers`
+    )
+
+    expect(response.status).toEqual(200)
+    expect(response.data.payment_providers).toEqual([])
+
+    await remoteLink.create([
+      {
+        [Modules.REGION]: {
+          region_id: region.id,
+        },
+        [Modules.PAYMENT]: {
+          payment_provider_id: "pp_system_default",
+        },
+      },
+    ])
+
+    response = await api.get(`/store/regions/${region.id}/payment-providers`)
+
+    expect(response.status).toEqual(200)
+    expect(response.data.payment_providers).toEqual([
+      expect.objectContaining({
+        id: "pp_system_default",
+      }),
+    ])
+  })
+})