feat: dependency graph visualization (Mermaid output)

[thomhurst]

Closes #4889. Adds --export-dependency-graph option.

[claude]

Code Review

Overall, this is a clean and well-structured implementation. The Mermaid export logic is clear, edge cases (no dependencies, no tests) are handled, and the opt-in CLI flag design is the right approach. Two issues were found.

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Issue 1 — Unnecessary allocation: array passed as List to a method that accepts IReadOnlyList

In ExportDependencyGraphIfRequestedAsync, allTests is AbstractExecutableTest[] (arrays implement IReadOnlyList<T> in .NET), so creating a new List<AbstractExecutableTest>(allTests) just to pass it to GenerateMermaidDiagram allocates an entire new collection unnecessarily.

TUnit/TUnit.Engine/Framework/TestRequestHandler.cs

Lines 95 to 98 in 02276ee

	var testList = new List<AbstractExecutableTest>(allTests);
	var mermaidContent = DependencyGraphExporter.GenerateMermaidDiagram(testList);

Fix: pass allTests directly:

var mermaidContent = DependencyGraphExporter.GenerateMermaidDiagram(allTests);

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Issue 2 — Mermaid label sanitization misses backticks in generic type names

test.Metadata.TestClassType.Name returns the CLR internal name for generic types, e.g. MyTests1forMyTests. Backtick ( ) is a special character in Mermaid (used for markdown-formatted labels) and is not currently escaped by SanitizeMermaidLabel. This produces malformed or visually broken node labels for any test class that uses generics.

Note: the rest of the TUnit codebase already handles this — TestIdentifierService, MetadataFilterMatcher, and ReflectionExtensions.GetFormattedName all strip the ``N` arity suffix before display.

TUnit/TUnit.Engine/Services/DependencyGraphExporter.cs

Lines 34 to 38 in 02276ee

	var className = test.Metadata.TestClassType.Name;
	var methodName = test.Metadata.TestMethodName;
	nodeLabels[testId] = $"{className}.{methodName}";
	testStates[testId] = test.State;

Fix: strip the arity suffix from the class name (consistent with what the rest of TUnit does) and escape backticks in SanitizeMermaidLabel:

// When building the label:
var className = test.Metadata.TestClassType.Name;
// Remove generic arity suffix: 'MyClass`2' -> 'MyClass'
var genericSuffixIndex = className.IndexOf('`');
if (genericSuffixIndex >= 0)
    className = className[..genericSuffixIndex];

// In SanitizeMermaidLabel, add:
.Replace("`", "#96;")