Problem

master CI has been failing on every push since ~January 2026. All PRs (including Dependabot bumps such as #425) inherit the failure, so nothing can go green. Exactly 6 tests fail on the Windows runner:

• RelationTests: AddRelationToUserAsync_ThrowsException_WhenRelationFieldIsNull, UpdateUserRelationAsync_ThrowsException_WhenRelationFieldIsNull, DeleteUserRelationAsync_ThrowsException_WhenRelationFieldIsNull, GetUserRelationsAsync_ThrowsException_WhenRelationFieldIsNull, AddRelationToUserAsync_ThrowsException_WhenRelatedObjectIsUnsaved
• UserTests.TestLogOut

All fail with the same Windows Schannel error:

System.Net.Http.HttpRequestException → IOException: The decryption operation failed
  → Win32Exception: The message received was unexpected or badly formatted

Root cause

These tests make real network requests that should have been mocked. new ServerConnectionData { Test = true } with no ServerURI defaults to the long-dead https://api.parse.com/1/ server (ParseClient.cs). On the Windows runner the TLS handshake against that endpoint fails; on other platforms it happens not to throw, which is why the failure was invisible locally.

Two separate mocking gaps let the calls escape:

1. RelationTests.SetUp created a MutableServiceHub with mocked controllers but never passed it to the ParseClient, so await user.SignUpAsync() in the test arrange hit the real network before the assertion was ever reached. (The *_WhenUserIsNull variants pass because they throw ArgumentNullException before any network call.)
2. UserTests.TestLogOut bound the user to the un-mocked SetUp client, so session revocation during logout (ParseUser.LogOutAsync → Services.RevokeSessionAsync) went to the network. It also used .CallBase() on an interface mock, which throws once the network no longer masks it.

Fix (test-only)

• RelationTests.SetUp: wire the mock hub into the client (new ParseClient(connectionData, hub)) and make TearDown null-safe for the resulting OrchestrationServiceHub.
• TestLogOut: user.Bind(client) so session revocation uses the mocked SessionController, and replace the invalid .CallBase() with .Returns(Task.CompletedTask).

No SDK/product code is changed.

Verification

Each fix was verified by first reproducing the failure deterministically (pointing the test client at an unreachable URI → Connection refused, mirroring the CI failure), then confirming the tests pass with that unreachable URI still set — proving the tests no longer depend on the network. Full suite:

Passed! - Failed: 0, Passed: 234, Skipped: 0, Total: 234

(Was Failed: 6, Passed: 228.)