fix(operators): serialize timeout terminal delivery

[ChrisPulman]

What kind of change does this PR introduce?

Bug fix for timeout operator concurrency.

What is the new behavior?

Timeout terminal delivery is serialized with source notifications so a timeout OnError cannot enter an observer concurrently with an in-flight OnNext.

Fixes #68.

What is the current behavior?

A timeout timer can emit OnError while the upstream OnNext path is still inside the downstream observer, allowing observer calls to overlap.

Checklist

• 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

Validation:

• dotnet build "tests/ReactiveUI.Primitives.Tests/ReactiveUI.Primitives.Tests.csproj" -f net10.0 --no-restore -v minimal
• dotnet test "tests/ReactiveUI.Primitives.Tests/ReactiveUI.Primitives.Tests.csproj" -f net10.0 --no-build -- --treenode-filter "/*/*/ExpireCoordinatorTests/*" --output Detailed
• mcp__mtpunittestmcp.get_solution_coverage was run, but no Cobertura coverage data was present under the solution directory.

A full multi-target test command was attempted first but exceeded the local timeout while the repository was under parallel issue validation load.

[glennawatson]

Verified against #68. The shared Lock now serializes value delivery with the timeout: OnNext/OnError/OnCompleted/EmitTimeout all check-and-set _done and forward under the gate, so a timeout OnError can no longer enter the observer concurrently with an in-flight OnNext. Dispose stays outside the lock. System.Threading.Lock is reentrant, so synchronous re-entrancy won't deadlock. Threaded test confirms it. LGTM.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.39%. Comparing base (d101d36) to head (1d54bea).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #84      +/-   ##
==========================================
+ Coverage   90.37%   90.39%   +0.02%     
==========================================
  Files         649      649              
  Lines       20228    20251      +23     
  Branches     2416     2419       +3     
==========================================
+ Hits        18281    18306      +25     
+ Misses       1564     1563       -1     
+ Partials      383      382       -1     

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[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.