Replace FluentAssertions with Shouldly for test assertions

[si618]

Summary

• Replace FluentAssertions with Shouldly assertion library across the entire codebase
• Migrate 50+ C# problem files with 140+ assertion patterns to more natural Shouldly syntax
• Maintain identical test behavior while improving readability and error messages

Changes Made

• Package Management: Remove FluentAssertions 7.2.0, add Shouldly 4.2.1
• Global Imports: Update GlobalUsings.cs to import Shouldly instead of FluentAssertions
• Assertion Migration: Convert patterns across all test files:
  • .Should().Be() → .ShouldBe()
  • .Should().BeTrue() → .ShouldBeTrue()
  • .Should().BeFalse() → .ShouldBeFalse()
  • .Should().BeEquivalentTo() → .ShouldBeEquivalentTo()
  • .Should().Throw<T>() → .ShouldThrow<T>()
• Code Cleanup: Remove FluentAssertions-specific AssertionScope usage
• Complex Objects: Use ShouldBeEquivalentTo() for ListNode and TreeNode comparisons
• Documentation: Update CLAUDE.md to reflect Shouldly usage

Test Results

• ✅ All 74 C# tests pass
• ✅ All 6 F# tests pass
• ✅ Build succeeds without errors
• ✅ Identical test behavior maintained

Benefits

• More Natural Syntax: Shouldly provides cleaner, more readable assertions
• Better Error Messages: Enhanced failure output with actual vs expected values
• Lighter Weight: Smaller package footprint than FluentAssertions
• Active Development: Strong community support and regular updates

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