Fix ProcessThreadTests.TestStartTimeProperty flakiness due to /proc visibility race

[Copilot]

TestStartTimeProperty was flaky (especially on Mono/Linux) because after a new LongRunning thread starts, its /proc/self/task/ entry may not be visible immediately. The previous code called p.Refresh() once then used Assert.Single, failing hard if the entry wasn't ready yet.

Description

Replace the single-shot refresh + Assert.Single with a bounded retry loop that re-calls p.Refresh() until the thread appears (up to 10 attempts, 100ms apart):

int newThreadId = GetCurrentThreadId();

ProcessThread newThread = null;
for (int i = 0; i < 10 && newThread is null; i++)
{
    if (i > 0) Thread.Sleep(100);
    p.Refresh();
    newThread = p.Threads.Cast<ProcessThread>().FirstOrDefault(t => t.Id == newThreadId);
}

Assert.True(newThread is not null, $"Thread with id {newThreadId} was not found after retrying.");
Assert.InRange(newThread.StartTime.ToUniversalTime(), curTime - allowedWindow, DateTime.Now.ToUniversalTime() + allowedWindow);

• Gets the thread ID before any refresh attempt
• Uses FirstOrDefault (not Assert.Single) during the search phase to avoid spurious failures
• Fails with a descriptive message if the thread still isn't visible after ~900ms of retries
• Preserves the exact-ID matching introduced in ProcessThreadTests.TestStartTimeProperty - Get spawned thread by ID from Process.Threads to avoid mismatch #104972 — if Mono returns wrong thread IDs, the retry will still fail and surface the real problem

Original prompt

---

This section details on the original issue you should resolve

<issue_title>ProcessThreadTests.TestStartTimeProperty failure in CI</issue_title>
<issue_description>```
System.Diagnostics.Tests.ProcessThreadTests.TestStartTimeProperty [FAIL]
Assert.Single() Failure: The collection did not contain any matching items
Expected: (predicate expression)
Collection: [System.Diagnostics.ProcessThread, System.Diagnostics.ProcessThread, System.Diagnostics.ProcessThread, System.Diagnostics.ProcessThread, System.Diagnostics.ProcessThread, ···]
Stack Trace:
//src/libraries/System.Diagnostics.Process/tests/ProcessThreadTests.cs(156,0): at System.Diagnostics.Tests.ProcessThreadTests.<>c__DisplayClass4_2.b__0()
//src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs(2396,0): at System.Threading.Tasks.Task.InnerInvoke()
//src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs(2384,0): at System.Threading.Tasks.Task.<>c.<.cctor>b__292_0(Object obj)
//src/libraries/System.Private.CoreLib/src/System/Threading/ExecutionContext.cs(179,0): at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location ---
//src/libraries/System.Private.CoreLib/src/System/Threading/ExecutionContext.cs(203,0): at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
//src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs(2342,0): at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)
--- End of stack trace from previous location ---
/_/src/libraries/System.Diagnostics.Process/tests/ProcessThreadTests.cs(149,0): at System.Diagnostics.Tests.ProcessThreadTests.TestStartTimeProperty()
--- End of stack trace from previous location ---

## Build Information
Build: https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=755830
Build error leg or test failing: System.Diagnostics.Tests.ProcessThreadTests.TestStartTimeProperty
Pull request: https://github.com/dotnet/runtime/pull/105519
<!-- Error message template  -->
## Error Message

Fill the error message using [step by step known issues guidance](https://github.com/dotnet/arcade/blob/main/Documentation/Projects/Build%20Analysis/KnownIssueJsonStepByStep.md).

<!-- Use ErrorMessage for String.Contains matches. Use ErrorPattern for regex matches (single line/no backtracking). Set BuildRetry to `true` to retry builds with this error. Set ExcludeConsoleLog to `true` to skip helix logs analysis. -->

```json
{
  "ErrorMessage": ["System.Diagnostics.Tests.ProcessThreadTests.TestStartTimeProperty [FAIL]","The collection did not contain any matching items"],
  "ErrorPattern": "",
  "BuildRetry": false,
  "ExcludeConsoleLog": false
}

Known issue validation

Build: 🔎 https://dev.azure.com/dnceng-public/public/_build/results?buildId=755830
Error message validated: [System.Diagnostics.Tests.ProcessThreadTests.TestStartTimeProperty [FAIL] The collection did not contain any matching items]
Result validation: ✅ Known issue matched with the provided build.
Validation performed at: 7/26/2024 12:06:54 AM UTC

Report

Build	Definition	Test	Pull Request
1342756	dotnet/runtime	System.Diagnostics.Tests.ProcessThreadTests.TestStartTimeProperty	#115996
1332497	dotnet/runtime	System.Diagnostics.Tests.ProcessThreadTests.TestStartTimeProperty	#125022
1326378	dotnet/runtime	System.Diagnostics.Tests.ProcessThreadTests.TestStartTimeProperty	#124923
1315488	dotnet/runtime	[System.Diagnostics.Tests.ProcessThreadTests.TestStartTimeProperty](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1315488&view=ms.vss-test-web.build...

• Fixes ProcessThreadTests.TestStartTimeProperty failure in CI #105526

---

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[Copilot]

Pull request overview

This PR reduces flakiness in ProcessThreadTests.TestStartTimeProperty by handling a race where a newly started thread may not immediately appear in Process.Threads (notably on Linux/Mono), replacing a single-shot refresh + Assert.Single with a bounded retry loop.

Changes:

• Remove the single p.Refresh() + Assert.Single(...) lookup for the spawned thread.
• Add a bounded retry loop that refreshes and searches for the thread by ID before asserting.
• Improve failure behavior by emitting a clearer assertion message when the thread never becomes visible.