fix: Tolerate missing directories when formatting from stdin

Src/CSharpier.Cli/CommandLineFormatter.cs

@@ -40,6 +40,15 @@ CancellationToken cancellationToken
                     filePath = directoryPath;
                     directoryPath = fileSystem.Path.GetDirectoryName(directoryPath);
                     ArgumentNullException.ThrowIfNull(directoryPath);
+
+                    // The directory from --stdin-path may not exist on disk.
+                    // Walk up to the nearest existing ancestor for config resolution.
+                    while (!fileSystem.Directory.Exists(directoryPath))
+                    {
+                        directoryPath = fileSystem.Path.GetDirectoryName(directoryPath);
+                        ArgumentNullException.ThrowIfNull(directoryPath);
+                    }
+
                     pathSupplied = true;
                 }
                 // otherwise someone is running this as a single command and not sending a path

Src/CSharpier.Cli/Options/OptionsProvider.cs

@@ -253,7 +253,10 @@ searchingDirectory is not null
             && !dictionary.TryGetValue(searchingDirectory.FullName, out result)
         )
         {
-            if (shouldConsiderDirectory(searchingDirectory.FullName))
+            if (
+                this.fileSystem.Directory.Exists(searchingDirectory.FullName)
+                && shouldConsiderDirectory(searchingDirectory.FullName)
+            )
             {
                 dictionary[searchingDirectory.FullName] = result = await createFileAsync(
                     searchingDirectory.FullName

Src/CSharpier.Tests/CommandLineFormatterTests.cs

@@ -781,6 +781,20 @@ public void Should_Format_StandardInput_And_Not_Consider_Gitignore_When_No_Path_
         result.OutputLines.First().Should().Be(FormattedClassContent);
     }
 
+    [Test]
+    public void Should_Format_StandardInput_When_StdinFilePath_Directory_Does_Not_Exist()
+    {
+        var context = new TestContext();
+        var result = Format(
+            context,
+            standardInFileContents: UnformattedClassContent,
+            directoryOrFilePaths: "NonExistent/SubDir/File.cs"
+        );
+
+        result.OutputLines.Should().ContainSingle();
+        result.OutputLines.First().Should().Be(FormattedClassContent);
+    }
+
     [Test]
     public void File_With_Mismatched_Line_Endings_In_Verbatim_String_Should_Pass_Validation()
     {