Build appended file targets per verification (#1885)

Appended files were stored as Targets, and a binary one holds a live stream. The
engine disposes the stream of every target it writes, so a second Verify with
the same settings instance failed with ObjectDisposedException, even though the
caller had handed over re-readable bytes or a file path. Settings are reused
routinely, since SettingsTask copies them per Verify call.

They are now stored as factories and built once per verification:
AppendContentAsFile(byte[]) opens a fresh MemoryStream over the same array, and
AppendFile(string) opens the file per verification instead of holding a handle
from the moment it was appended. Existence is still checked eagerly, so a
missing path is still reported where it was passed.

AppendFile(Stream) is unchanged and still single use: the stream is owned by the
caller and cannot be replayed. That is now documented on the overload.

Author: SimonCropp

src/Verify.Tests/Converters/InstanceFileAppenderTests.AppendFileSettingsReuse_first#sample.verified.png

@@ -0,0 +1 @@
+First

src/Verify.Tests/Converters/InstanceFileAppenderTests.AppendFileSettingsReuse_first.verified.txt

@@ -0,0 +1 @@
+Second

src/Verify.Tests/Converters/InstanceFileAppenderTests.AppendFileSettingsReuse_second#sample.verified.png

@@ -0,0 +1 @@
+


src/Verify.Tests/Converters/InstanceFileAppenderTests.AppendFileSettingsReuse_second.verified.txt

@@ -0,0 +1 @@
+First

src/Verify.Tests/Converters/InstanceFileAppenderTests.BinaryBytesSettingsReuse_first#appendedBytes.verified.bin

@@ -0,0 +1 @@
+


src/Verify.Tests/Converters/InstanceFileAppenderTests.BinaryBytesSettingsReuse_first.verified.txt

@@ -0,0 +1 @@
+Second

src/Verify.Tests/Converters/InstanceFileAppenderTests.BinaryBytesSettingsReuse_second#appendedBytes.verified.bin

@@ -55,4 +55,31 @@ public Task TextBytesFluent() =>
     public Task TextStreamFluent() =>
         Verify("Foo")
             .AppendFile(new MemoryStream("appendedFile"u8.ToArray()));
+
+    // The engine disposes the stream of every target it writes, so an appended binary file
+    // held as a live stream was dead after the first verification. Both of these are backed
+    // by something re-readable, so reusing the settings has to work.
+    [Fact]
+    public async Task BinaryBytesSettingsReuse()
+    {
+        var reused = new VerifySettings();
+        reused.AppendContentAsFile(new byte[] {1, 2, 3}, "bin", "appendedBytes");
+
+        await Verify("First", reused)
+            .UseMethodName("BinaryBytesSettingsReuse_first");
+        await Verify("Second", reused)
+            .UseMethodName("BinaryBytesSettingsReuse_second");
+    }
+
+    [Fact]
+    public async Task AppendFileSettingsReuse()
+    {
+        var reused = new VerifySettings();
+        reused.AppendFile("sample.png");
+
+        await Verify("First", reused)
+            .UseMethodName("AppendFileSettingsReuse_first");
+        await Verify("Second", reused)
+            .UseMethodName("AppendFileSettingsReuse_second");
+    }
 }

src/Verify.Tests/Converters/InstanceFileAppenderTests.BinaryBytesSettingsReuse_second.verified.txt

@@ -28,6 +28,19 @@ public static void BadParametersText(string value, [CallerArgumentExpression(nam
         }
     }
 
+    /// <summary>
+    /// Kept as a fail fast for APIs that record a path and read it later, so a missing
+    /// file is reported where the caller passed it rather than at verification time.
+    /// </summary>
+    public static void FileExists(string path, [CallerArgumentExpression(nameof(path))] string argumentName = "")
+    {
+        Ensure.NotNullOrEmpty(path, argumentName);
+        if (!File.Exists(path))
+        {
+            throw new FileNotFoundException($"File not found. Path: {path}", path);
+        }
+    }
+
     static char[] invalidPathChars = Path
         .GetInvalidPathChars()
         .Concat(invalidFileChars.Except(['/', '\\', ':']))