fix: serialize MTP server-mode concurrent sessions to prevent stochastic 0-result runs

[Copilot]

Multiple concurrent testing/runTests RPCs to one long-lived TUnit process race on shared static state (GlobalContext.Current, AppDomain.UnhandledException handlers, Trace.Listeners), producing stochastic 0-result sessions. Sequential execution is deterministic and correct; concurrency introduces variance.

Changes

• Serialize ExecuteRequestAsync — added SemaphoreSlim(1, 1) in TUnitTestFramework so concurrent RPCs queue and execute sequentially. The perf win from MTP server mode is avoiding process startup overhead, not intra-process parallelism.

• Make TUnitInitializer.Initialize idempotent — static volatile flags prevent repeated AppDomain.UnhandledException handler registration and Trace.Listeners insertion across sessions.

• Add multi-session RPC tests — validates both sequential (3×) and concurrent (3× parallel) runTests RPCs against the same server produce consistent, complete results.

// TUnitTestFramework.cs
private readonly SemaphoreSlim _executeRequestLock = new(1, 1);

public async Task ExecuteRequestAsync(ExecuteRequestContext context)
{
    await _executeRequestLock.WaitAsync(context.CancellationToken);
    try { /* ... */ }
    finally
    {
        _executeRequestLock.Release();
        context.Complete();
    }
}

[codacy-production]

Not up to standards ⛔

🔴 Issues 1 high

Alerts:
⚠ 1 issue (≤ 0 issues of at least minor severity)

Results:
1 new issue

Category	Results
ErrorProne	1 high

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🟢 Metrics 4 complexity

Metric	Results
Complexity	4

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[thomhurst]

@copilot this is a band aid - we should correctly scope static state to either instance level or async local. Concurrent runs is a valid pattern.

[Copilot]

@copilot this is a band aid - we should correctly scope static state to either instance level or async local. Concurrent runs is a valid pattern.

Agreed — removed the SemaphoreSlim and converted GlobalContext.Current to AsyncLocal<GlobalContext?> (matching the pattern already used by TestSessionContext, BeforeTestDiscoveryContext, TestDiscoveryContext). A static DefaultInstance serves source-gen module initializers that run before any session sets the value. Concurrent sessions now each get their own context without races.