Apply SonarQube code quality improvements

[Copilot]

SonarQube Cleanup Improvements

• Replace collection initialization new() with {} in source files
  • TraceMatrix.cs: _testResults field
  • Requirement.cs: Tests and Children properties
  • Context.cs: RequirementsFiles and TestFiles properties
  • Requirements.cs: _includedFiles and _allRequirements fields
  • Section.cs: Requirements and Sections properties
  • TraceMatrix.cs: List initialization in TryGetValue block
• Use ArgumentNullException.ThrowIfNull instead of manual null checks
  • TraceMatrix.cs: Constructor null check for requirements parameter
• Mark CollectAllTests method as static in TraceMatrix.cs
• Replace Dictionary ContainsKey + indexer with TryGetValue in TraceMatrix.cs
• Replace StringAssert.Contains with Assert.Contains in test files
  • RequirementsExportTests.cs
  • RequirementsReadTests.cs
  • TraceMatrixReadTests.cs
  • TraceMatrixExportTests.cs
  • TraceMatrixTests.cs
  • ContextTests.cs
  • ProgramTests.cs
  • RequirementsParsingTests.cs
• Replace constant array argument with static readonly field in TraceMatrixExportTests.cs

All changes complete. Build succeeded with 0 warnings and 0 errors. All 127 tests passed.

Original prompt

Please perform additional cleanups identified by SonarQube - specifically things like:

• Collection Initialization can be simplified - recommending "{}" over "new()"
• Use of ArgumentNullException.ThrowIfNull
• Member 'CollectAllTests' does not access instance data and can be marked as static
• Prefer a 'TryGetValue' call over a Dictionary indexer access guarded by a 'ContainsKey' check to avoid double lookup
• Use 'Assert.Contains' instead of 'StringAssert.Contains'
• Prefer 'static readonly' fields over constant array arguments if the called method is called repeatedly and is not mutating the passed array

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