[iammukeshm]

Closes #1457.

All 16 test projects move from xunit 2.9.3 to xUnit v3 (xunit.v3.mtp-off 4.0.2).

Why mtp-off. The default xunit.v3 package turns on Microsoft.Testing.Platform, and then the .NET 10 SDK refuses VSTest-style dotnet test unless global.json opts into the new runner. The template doesn't ship global.json, so every scaffolded project would lose dotnet test. mtp-off keeps VSTest: dotnet test, --filter, the coverlet collector and coverage.runsettings all work as before, and CI's test steps don't change. Moving to MTP can be a separate change.

The migration itself:

• Test projects are executables now (<OutputType>Exe</OutputType>).
• IAsyncLifetime returns ValueTask in v3. FshWebApplicationFactory and MiddlewareWebApplicationFactory now override WebApplicationFactory.DisposeAsync() instead of hiding it with new, so there's a single disposal path. The five test classes with their own lifetimes are updated too.
• xUnit1051 is off for src/Tests (src/.editorconfig, with the reason). I tried its code fix first. It also adds TestContext.Current.CancellationToken to NSubstitute setups and Received() checks, which changes what the mock matches, because handlers under test get CancellationToken.None. In the integration tests it changes about 1,900 lines and adds that token to almost every one. The suite has no test timeouts for the token to serve.

Hidden test-order dependencies. v3 orders tests differently, and that exposed four places where tests relied on running in a particular order. The first full run had 189 failures. All four fixes are in the test harness; no production code changes.

1. WithWebHostBuilder hosts (webhook tests) replace two process-wide statics, Hangfire's JobStorage.Current and the static Audit publisher, and leave them pointing at their own storage and channel after disposal. Later enqueues threw ObjectDisposedException (most of the 189), and request audits went into a channel nobody reads. The new PrimaryHostStaticsRestorer points both back at the shared host whenever a host stops.
2. TenantProvisioningFailureTests and RetryTenantProvisioningTests left tenants with an unreachable database in the store, so the legacy public-key migration job (which walks every tenant) failed in StorageVisibilityRootTests. They now remove their tenants through a new FshWebApplicationFactory.RemoveTenantsAsync.
3. The audit tenant-isolation poll read only the newest 100 rows, but a new tenant can have more than 100 EntityChange seed rows, depending on what earlier tests created. It now excludes EntityChange.
4. The "within the auth limit" rate-limit test shared TestServer's null-IP partition with the test that exhausts it. It now uses its own client IP.

Tested:

• dotnet build src/FSH.Starter.slnx: 0 warnings, 0 errors.
• dotnet test src/FSH.Starter.slnx: 2,225/2,225 passing. Every project's count matches main's last Backend CI run: Integration 805, Middleware 7, Framework 373, Identity 352, and so on.
• --collect "XPlat Code Coverage" --settings coverage.runsettings still produces coverage.cobertura.xml.

Docs: fullstackhero/docs PR to follow (testing pages and changelog).