Upgrade major dependencies

[drewburlingame]

• FluentAssertions 7.0.0 → 8.0.0
• Microsoft.NET.Test.Sdk 17.12.0 → 18.0.0
• codecov-action v4 → v5

All tests passing (946 passed, 4 skipped).

Closes #530, #535, #542

Description

Related Issue

Fixes #(issue number)

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Code refactoring
• Test improvements
• CI/CD improvements

Changes Made

Testing

New Tests Added

• Unit tests added
• Feature tests added
• Integration tests added
• No tests needed (explain why)

Test Results

# Paste test results here
dotnet test output

Manual Testing Performed

Documentation

• Updated XML documentation comments
• Updated README if needed
• Updated documentation site (docs/) if needed
• Added/updated code examples in DocExamples
• No documentation changes needed

Breaking Changes

• This PR introduces breaking changes
• Migration guide updated (if applicable)

Breaking changes description:

Checklist

• My code follows the project's coding standards
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• 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
• Any dependent changes have been merged and published

Screenshots (if applicable)

Additional Context

Reviewer Notes