chore: add GlobalUsings.cs with project-wide using directives in data structures tests

[KaranChadha10]

Description

This pull request contains the following changes

• Converted block-scoped namespaces to file-scoped namespaces for consistency in Data Structures Tests folder
• Removed redundant/unused global using directives to improve code clarity
• Aligned code formatting with project conventions and C# 10+ style guidelines

Issue link: #520

• I have performed a self-review of my code
• My code follows the style guidelines of this project
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Comments in areas I changed are up to date
• I have added comments to hard-to-understand areas of my code
• I have made corresponding changes to the README.md

[Copilot]

Pull Request Overview

This pull request adds a GlobalUsings.cs file with project-wide using directives and removes redundant using statements from test files across the DataStructures.Tests project. This modernizes the codebase to use C# 10+ global using features for better code organization and reduces repetitive using declarations.

• Added GlobalUsings.cs with commonly used namespaces declared globally
• Removed redundant using statements from all test files that are now covered by global usings
• Minor code formatting improvement in one test method

Reviewed Changes

Copilot reviewed 43 out of 43 changed files in this pull request and generated 2 comments.

File	Description
DataStructures.Tests/GlobalUsings.cs	Introduces global using directives for common namespaces (System, NUnit.Framework, FluentAssertions, etc.)
Multiple test files	Removes redundant using statements now covered by global usings
DataStructures.Tests/Hashing/HashTableTests.cs	Minor formatting improvement to lambda expression

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.20%. Comparing base (ea17d81) to head (2a4571c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #526      +/-   ##
==========================================
+ Coverage   95.15%   95.20%   +0.04%     
==========================================
  Files         272      272              
  Lines       10875    10875              
  Branches     1529     1529              
==========================================
+ Hits        10348    10353       +5     
+ Misses        400      397       -3     
+ Partials      127      125       -2     

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[siriak]

Re-request a review when the comments are addressed

[Copilot]

Pull Request Overview

Copilot reviewed 43 out of 43 changed files in this pull request and generated no new comments.

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[siriak]

Looks good, thanks!