Add custom_attributes field to OrganizationMembership (#524)

* Add idp_attributes field to OrganizationMembership

- Add idp_attributes: dict[str, Any] to OrganizationMembership model
- Update MockOrganizationMembership fixture
- All organization_membership tests pass

This change adds support for IDP custom attributes on organization
memberships, which are sourced from the identity provider and stored
as customAttributes in the API.

Related to workos/workos PR #50470

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

* Fix Python 3.8 compatibility for idp_attributes type

Use Dict[str, Any] instead of dict[str, Any] for Python 3.8 compatibility.
The lowercase dict syntax is only available in Python 3.9+.

Fixes issue identified by Greptile bot review.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

* Rename idp_attributes to custom_attributes

* Use Mapping instead of Dict for read-only custom_attributes

---------

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: ajworkos

src/workos/types/user_management/organization_membership.py

@@ -1,4 +1,4 @@
-from typing import Literal, Sequence, Optional
+from typing import Any, Literal, Mapping, Optional, Sequence
 from typing_extensions import TypedDict
 
 from workos.types.workos_model import WorkOSModel
@@ -21,5 +21,6 @@ class OrganizationMembership(WorkOSModel):
     role: OrganizationMembershipRole
     roles: Optional[Sequence[OrganizationMembershipRole]] = None
     status: LiteralOrUntyped[OrganizationMembershipStatus]
+    custom_attributes: Mapping[str, Any]
     created_at: str
     updated_at: str

tests/utils/fixtures/mock_organization_membership.py

@@ -13,6 +13,7 @@ def __init__(self, id):
             organization_id="org_67890",
             status="active",
             role={"slug": "member"},
+            custom_attributes={},
             created_at=now,
             updated_at=now,
         )