MPT-18902 E2E API client exception with the seller tests (#224)

https://softwareone.atlassian.net/wiki/spaces/mpt/pages/5243175002/Seller+Object

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-->
Closes [MPT-18902](https://softwareone.atlassian.net/browse/MPT-18902)

- tests/e2e/accounts/conftest.py: currencies fixture now returns a list
of currency objects (dicts with keys value, billingEnabled, isDefault)
instead of a simple list of strings.
- tests/e2e/accounts/sellers/conftest.py: seller_factory's inner _seller
sets the seller dictionary key "currency" (singular) to the provided
currencies value; the factory parameter remains named currencies but the
returned data shape changed from a "currencies" key to "currency"
containing the structured list.
- No changes to fixture names or function signatures; only the returned
data structures were modified.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

[MPT-18902]:
https://softwareone.atlassian.net/browse/MPT-18902?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: d3rky

tests/e2e/accounts/conftest.py

@@ -23,7 +23,10 @@ def account_icon(logo_fd):
 
 @pytest.fixture
 def currencies():
-    return ["USD", "EUR"]
+    return [
+        {"value": "EUR", "billingEnabled": False, "isDefault": False},
+        {"value": "USD", "billingEnabled": True, "isDefault": True},
+    ]
 
 
 @pytest.fixture

tests/e2e/accounts/sellers/conftest.py

@@ -22,7 +22,7 @@ def _seller(
                 "postCode": "12345",
                 "country": "US",
             },
-            "currencies": currencies,
+            "currency": currencies,
             "externalId": external_id,
         }