refactor: implement two-phase architecture for xUnit migration code fixer

TUnit.Analyzers.CodeFixers/Base/MigrationContext.cs

@@ -0,0 +1,60 @@
+using Microsoft.CodeAnalysis.Text;
+
+namespace TUnit.Analyzers.CodeFixers.Base;
+
+/// <summary>
+/// Tracks migration progress and failures during code fix execution.
+/// Enables partial success by recording what failed without aborting the entire migration.
+/// Failures are surfaced via TODO comments in the generated code.
+/// </summary>
+public class MigrationContext
+{
+    public List<MigrationFailure> Failures { get; } = new();
+
+    /// <summary>
+    /// Records a failure during a migration step.
+    /// The failure will be reported as a TODO comment in the migrated file.
+    /// </summary>
+    public void RecordFailure(string step, Exception ex, TextSpan? span = null)
+    {
+        var failure = new MigrationFailure(
+            Step: step,
+            Description: ex.Message,
+            Span: span,
+            OriginalCode: null,
+            StackTrace: ex.StackTrace);
+
+        Failures.Add(failure);
+    }
+
+    /// <summary>
+    /// Records a failure during item-level conversion (e.g., a specific assertion).
+    /// Includes the original code snippet for context in the TODO comment.
+    /// </summary>
+    public void RecordItemFailure(string step, string originalCode, Exception ex, TextSpan? span = null)
+    {
+        var failure = new MigrationFailure(
+            Step: step,
+            Description: ex.Message,
+            Span: span,
+            OriginalCode: originalCode,
+            StackTrace: ex.StackTrace);
+
+        Failures.Add(failure);
+    }
+
+    /// <summary>
+    /// Returns true if any failures were recorded.
+    /// </summary>
+    public bool HasFailures => Failures.Count > 0;
+}
+
+/// <summary>
+/// Represents a single migration failure with context for debugging.
+/// </summary>
+public record MigrationFailure(
+    string Step,
+    string Description,
+    TextSpan? Span = null,
+    string? OriginalCode = null,
+    string? StackTrace = null);