chore: Serialize test collections to avoid flaky initialization timeouts

[kinyoklion]

Disables xUnit cross-collection parallelism for the test project so provider initialization tests stop flaking.

• ClientIntegrationTests and ProviderTests both drive the process-global Api.Instance and use Thread.Sleep plus 100 ms timers inside [Fact(Timeout = 5000)]; running those classes concurrently starves the thread pool and produces spurious Test execution timed out after 5000 milliseconds failures on loaded runners.
• Adds xunit.runner.json with parallelizeTestCollections: false, copied to the output directory from the csproj.
• Observed failure: the net6.0 leg of ci-build (macos-latest) on fix: Validate privateAttributes in evaluation context conversion #70 failed ClientIntegrationTests.ItHandlesValidInitializationWhenClientIsReadyAfterADelay after 1 ms of test body time, while the identical net8.0 leg in the same job passed.

Requirements

• I have added test coverage for new or changed functionality (test-infrastructure only; no behavior change)
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions (verified locally on net8.0; net6.0/net471 validated by CI)

Details

Related issues

None. Surfaced while working on #69 and #70.

Describe the solution you've provided

These initialization tests are inherently global: they register handlers on Api.Instance and assert on event counts after fixed sleeps. Serializing collections removes the cross-class interference and the thread-pool contention that makes xUnit's Timeout fire even though the test body does almost no work. Tests inside a single class already ran sequentially, so this only changes inter-class scheduling; total suite time is unaffected in practice (~1s).

Describe alternatives you've considered

• Rewriting the tests to avoid Api.Instance and Thread.Sleep — larger change, and the OpenFeature API surface is intentionally a singleton, so some global usage is unavoidable.
• Raising the Timeout values — hides the contention rather than removing it.

Additional context

Local verification:

dotnet test test/LaunchDarkly.OpenFeature.ServerProvider.Tests -f net8.0
Passed!  - Failed: 0, Passed: 68, Skipped: 0, Total: 68

Link to Devin session: https://app.devin.ai/sessions/9f0be899af7842e0b6a7bfc6229084f6
Open in Devin Desktop: https://app.devin.ai/desktop/session/9f0be899af7842e0b6a7bfc6229084f6?variant=devin
Requested by: @kinyoklion

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring