fix: ensure thread-safe initialization of console line buffers

Addresses post-merge feedback from PR #4549 regarding thread-safety
concerns in console buffer lazy initialization.

Replaced nullable StringBuilder fields with Lazy<StringBuilder> to
guarantee thread-safe initialization when multiple threads access
GetConsoleStdOutLineBuffer() or GetConsoleStdErrLineBuffer() concurrently.

The previous implementation using ??= was not thread-safe and could
result in multiple StringBuilder instances being created before
assignment completes, undermining per-context buffer isolation.

Benefits:
- Thread-safe by default with Lazy<T>
- No manual double-checked locking needed
- Cleaner, more idiomatic C# code
- AOT-compatible

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: thomhurst

TUnit.Core/Context.cs

@@ -29,8 +29,9 @@ TestContext.Current as Context
 
     // Console interceptor line buffers for partial writes (Console.Write without newline)
     // These are stored per-context to prevent output mixing between parallel tests
-    private StringBuilder? _consoleStdOutLineBuffer;
-    private StringBuilder? _consoleStdErrLineBuffer;
+    // Using Lazy<T> for thread-safe initialization
+    private readonly Lazy<StringBuilder> _consoleStdOutLineBuffer = new(() => new StringBuilder());
+    private readonly Lazy<StringBuilder> _consoleStdErrLineBuffer = new(() => new StringBuilder());
     private readonly object _consoleStdOutBufferLock = new();
     private readonly object _consoleStdErrBufferLock = new();
 
@@ -43,12 +44,12 @@ TestContext.Current as Context
     // Internal accessors for console interceptor line buffers with thread safety
     internal (StringBuilder Buffer, object Lock) GetConsoleStdOutLineBuffer()
     {
-        return (_consoleStdOutLineBuffer ??= new StringBuilder(), _consoleStdOutBufferLock);
+        return (_consoleStdOutLineBuffer.Value, _consoleStdOutBufferLock);
     }
 
     internal (StringBuilder Buffer, object Lock) GetConsoleStdErrLineBuffer()
     {
-        return (_consoleStdErrLineBuffer ??= new StringBuilder(), _consoleStdErrBufferLock);
+        return (_consoleStdErrLineBuffer.Value, _consoleStdErrBufferLock);
     }
 
     internal Context(Context? parent)