Add race condition examples to PLINQ shared memory pitfalls section

[Copilot]

The "Avoid writing to shared memory locations" section in the PLINQ documentation lacked concrete examples demonstrating race conditions.

Changes

• Added race condition example showing unsafe parallel writes to shared variable producing incorrect results
• Added correct approach using thread-safe Sum() aggregation
• Created compilable snippets in both C# and VB with project files
• Enhanced explanation of why total += n fails (non-atomic read-modify-write causing lost updates)

Example

// UNSAFE: Race condition
int total = 0;
numbers.AsParallel().ForAll(n => total += n);
// Result: unpredictable, less than expected 49,995,000

// SAFE: Thread-safe aggregation
int total = numbers.AsParallel().Sum();
// Result: always 49,995,000

The documentation now references Aggregate() and ConcurrentBag<T> as alternative safe patterns.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Consider adding more examples</issue_title>
<issue_description>Specially under the "Avoid writing to shared memory locations". An example that shows a race condition would be really good to understand this concept.

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Document Details

⚠ Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.

• ID: e37b6593-8529-3660-9d33-67be0f074834
• Version Independent ID: dbb274ac-6205-26ff-8017-aeccf772888a
• Content: Potential Pitfalls with PLINQ
• Content Source: docs/standard/parallel-programming/potential-pitfalls-with-plinq.md
• Product: dotnet
• Technology: dotnet-standard
• GitHub Login: @BillWagner
• Microsoft Alias: dotnetcontent</issue_description>

Comments on the Issue (you are @copilot in this section)

@dotnet-bot This issue has been closed as part of the issue backlog grooming process outlined in #22351.

That automated process may have closed some issues that should be addressed. If you think this is one of them, reopen it with a comment explaining why. Tag the @dotnet/docs team for visibility.</comment_new>
<comment_new>@Youssef1313
@BillWagner Consider re-opening this one.</comment_new>

• Fixes Consider adding more examples #16034

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