fix(build): hold the coverage collector at 18.10.0

[glennawatson]

What kind of change does this PR introduce?

Build fix. One version pin, no product or test source changes.

What is the new behavior?

A solution-wide run finishes cleanly, and the coverage it reports is complete again.

• Microsoft.Testing.Extensions.CodeCoverage is held at 18.10.0.

• Union coverage over a full dotnet test --solution --coverage run, same commit, only the collector version differing:

  collector	exit	line coverage	branch coverage
  18.10.0	0	12820/13537 = 94.703%	1051/1334 = 78.786%
  18.11.0	7	12097/13537 = 89.362%	1005/1334 = 75.337%

  The aborting host never reconciles its buffers, so its data is dropped and the reported number silently falls by about 5.3 points of line coverage. The pin recovers it.

• The pin carries a comment describing the failure, so the next bump gets checked against it rather than taken on trust.

What is the current behavior?

A test host that has run every test successfully aborts during session teardown, and the run reports exit code 7:

Unhandled exception. System.Threading.AbandonedMutexException: The wait completed due to an abandoned mutex.
   at Microsoft.CodeCoverage.Core.Threading.Mutex.WaitOne(TimeSpan timeout)
   at Microsoft.CodeCoverage.Instrumentation.SharedBufferReconciler.Reconcile(...)
   at Microsoft.Testing.Extensions.CodeCoverage.TestingPlatformCoverageDynamicTestSessionLifetimeHandler.OnTestSessionFinishingAsync(...)

SharedBufferReconciler waits on a cross-process mutex to reconcile coverage buffers. A solution-wide run passes that mutex between many concurrently running hosts, so when one exits while still owning it the next waiter is handed an AbandonedMutexException, which the collector does not handle. The exception goes unhandled, the process aborts with SIGABRT, and the platform maps that to exit code 7.

It lands on whichever assembly loses the race rather than on a particular test, which is why a single arbitrary assembly went red while its near-identical sibling passed in the same run. All 34300 tests pass either way; the crash is purely in teardown.

18.11.0 is the newest published version, so there is no later release to move forward to.

What might this PR break?

• Nothing shipped. Microsoft.Testing.Extensions.CodeCoverage is test-only tooling and is not a dependency of any published package.
• Coverage collection is unchanged in shape; 18.10.0 emits the same cobertura the CI coverage job already consumes, and reports more of it.
• Microsoft.Testing.Platform.MSBuild deliberately stays at 2.4.0. It appears in the stack only as the calling frame; the defect is in the collector.

Checklist

• I have read the Contribute guide
• Tests have been added or updated (for bug fixes / features)
• Docs have been added or updated (for bug fixes / features)
• Changes target the main branch
• PR title follows Conventional Commits

Additional information

The crash needs the whole solution running concurrently, not one assembly: looping a single test project 25 times never reproduced it, while a full --coverage solution run reproduced exit 7 on Linux as well. It is a race across hosts, not something specific to the macOS runner.

The stack is not visible through --log-failed, which shows only the runner's exit code; it is in the full job log.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.92%. Comparing base (6a2ef8a) to head (56333ae).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4453   +/-   ##
=======================================
  Coverage   93.92%   93.92%           
=======================================
  Files         359      359           
  Lines       15105    15105           
  Branches     1568     1568           
=======================================
  Hits        14188    14188           
  Misses        674      674           
  Partials      243      243           

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud