Add organization member management APIs

[PatriQ94]

Summary

This PR adds organization member management endpoints to the IKcOrganizations API and fixes a serialization bug in the organization models that prevents organization operations from working against Keycloak 26+.

Bug Fix

KcOrganization and KcOrganizationDomain models used System.Text.Json attributes ([JsonPropertyName]), but the HTTP client base serializes request bodies with Newtonsoft.Json (JsonConvert.SerializeObject), which ignores those attributes. This caused property names to serialize as PascalCase (e.g., "Name" instead of "name"), resulting in 400 Bad Request from Keycloak:

Invalid json representation for OrganizationRepresentation. Unrecognized field "Name"

Fixed by switching the two organization model files to use [JsonProperty] from Newtonsoft.Json, consistent with all other models in the library.

New Features

Added 9 organization member management methods to IKcOrganizations:

Method	HTTP	Endpoint
AddMemberAsync	POST	/{realm}/organizations/{id}/members
GetMembersAsync	GET	/{realm}/organizations/{id}/members
GetMembersCountAsync	GET	/{realm}/organizations/{id}/members/count
GetMemberAsync	GET	/{realm}/organizations/{id}/members/{memberId}
RemoveMemberAsync	DELETE	/{realm}/organizations/{id}/members/{memberId}
GetMemberOrganizationsAsync	GET	/{realm}/organizations/{id}/members/{memberId}/organizations
GetUserOrganizationsAsync	GET	/{realm}/organizations/members/{userId}/organizations
InviteExistingUserAsync	POST	/{realm}/organizations/{id}/members/invite-existing-user
InviteUserAsync	POST	/{realm}/organizations/{id}/members/invite-user

Added KcOrganizationMemberFilter model for paginated member queries (supports first, max, search, exact, membershipType).

Files Changed

• KcOrganization.cs, KcOrganizationDomain.cs — [JsonPropertyName] → [JsonProperty] (bug fix)
• IKcOrganizations.cs — 9 new method signatures
• KcOrganizations.cs — 9 new method implementations
• KcOrganizationMemberFilter.cs — new filter model
• KcOrganizationMemberHappyPathTests.cs — integration tests covering the full member lifecycle

Testing

All 21 organization tests pass against Keycloak 26.5.1 (existing 10 + new 11, with InviteExistingUser skipped when SMTP is not configured).

[PatriQ94]

@hasnimehdi91 Review please 🙏