fix: Convert parallel-letter-frequency exercise to use async/await

[KaranChadha10]

Summary

Converts the parallel-letter-frequency exercise from using PLINQ (AsParallel()) to proper async/await patterns with Task.Run and Task.WhenAll.

Changes Made

• Method signature: Changed Calculate to return Task<Dictionary<char, int>> instead of Dictionary<char, int>
• Tests: Updated all test methods to use async Task and await the Calculate method
• Implementation: Replaced AsParallel().Aggregate() with Task.Run + Task.WhenAll for better async/parallel demonstration
• Code structure: Separated concerns with dedicated helper methods for counting and merging

Issue link: #1894

[github-actions]

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[github-actions]

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[ErikSchierboom]

@KaranChadha10 CI is failing

[KaranChadha10]

@ErikSchierboom sure let me check.

[KaranChadha10]

@ErikSchierboom I've now updated the code. The checks should be passing now.

Please let me know if there's anything else that needs adjustment!

[ErikSchierboom]

Could you re-add the skips on all but the first test?

[KaranChadha10]

Done, @ErikSchierboom kindly review, thanks.

[ErikSchierboom]

Thanks!