Mono: Add source generator tests for external assembly script paths

Tests ScriptPathAttributeGenerator for multi-assembly scenarios:
- ExternalAssemblyInTree: source inside Godot project gets res:// path
- ExternalAssemblyOutOfTree: source outside project gets csharp:// path

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: matelq

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Tests/ScriptPathAttributeGeneratorTests.cs

@@ -1,3 +1,6 @@
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp.Testing;
+using Microsoft.CodeAnalysis.Testing;
 using Microsoft.CodeAnalysis.Text;
 using System;
 using System.Collections.Generic;
@@ -22,6 +25,29 @@ private static (string, SourceText) MakeAssemblyScriptTypesGeneratedSource(IColl
         );
     }
 
+    /// <summary>
+    /// Creates a verifier with a custom GodotProjectDir, for testing out-of-tree source paths.
+    /// </summary>
+    private static CSharpSourceGeneratorVerifier<ScriptPathAttributeGenerator>.Test MakeVerifierWithCustomProjectDir(
+        string godotProjectDir, string assemblyName = "TestProject")
+    {
+        var verifier = new CSharpSourceGeneratorVerifier<ScriptPathAttributeGenerator>.Test();
+
+        verifier.TestState.AnalyzerConfigFiles.Add(("/.globalconfig", $"""
+        is_global = true
+        build_property.GodotProjectDir = {godotProjectDir}
+        """));
+
+        verifier.SolutionTransforms.Add((Solution solution, ProjectId projectId) =>
+        {
+            Project project = solution.GetProject(projectId)!
+                .WithAssemblyName(assemblyName);
+            return project.Solution;
+        });
+
+        return verifier;
+    }
+
     [Fact]
     public async Task ScriptBoilerplate()
     {
@@ -78,4 +104,45 @@ public async Task NamespaceMultipleClassesSameName()
         verifier.TestState.GeneratedSources.Add(MakeAssemblyScriptTypesGeneratedSource(new string[] { "global::NamespaceA.SameName", "global::NamespaceB.SameName" }));
         await verifier.RunAsync();
     }
+
+    /// <summary>
+    /// Tests that an in-tree external assembly script (source inside Godot project dir)
+    /// gets a normal res:// path.
+    /// </summary>
+    [Fact]
+    public async Task ExternalAssemblyInTree()
+    {
+        var verifier = CSharpSourceGeneratorVerifier<ScriptPathAttributeGenerator>.MakeVerifier(
+            new string[] { "ExternalScript.cs" },
+            new string[] { "ExternalModule.ExternalScript_ScriptPath.generated.cs" }
+        );
+        verifier.TestState.GeneratedSources.Add(
+            MakeAssemblyScriptTypesGeneratedSource(new string[] { "global::ExternalModule.ExternalScript" }));
+        await verifier.RunAsync();
+    }
+
+    /// <summary>
+    /// Tests that an out-of-tree external assembly script (source outside Godot project dir)
+    /// gets a synthetic csharp:// path instead of an invalid res://../ path.
+    /// </summary>
+    [Fact]
+    public async Task ExternalAssemblyOutOfTree()
+    {
+        // Set GodotProjectDir to a subdirectory that won't contain the source file.
+        // The Roslyn test framework resolves source paths relative to CWD. By setting
+        // GodotProjectDir to a non-existent subdirectory, the computed relative path
+        // will start with "../", triggering the csharp:// synthetic path logic.
+        string deepProjectDir = Path.Combine(Constants.ExecutingAssemblyPath, "nonexistent", "godot_project");
+
+        var verifier = MakeVerifierWithCustomProjectDir(deepProjectDir, "ExternalLib");
+
+        string source = File.ReadAllText(Path.Combine(Constants.SourceFolderPath, "ExternalScript.cs"));
+        verifier.TestState.Sources.Add(("ExternalScript.cs", SourceText.From(source)));
+
+        // Skip exact source check because SourceFile contains a machine-dependent absolute path.
+        verifier.TestBehaviors |=
+            Microsoft.CodeAnalysis.Testing.TestBehaviors.SkipGeneratedSourcesCheck;
+
+        await verifier.RunAsync();
+    }
 }

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Tests/TestData/GeneratedSources/ExternalModule.ExternalScript_ScriptPath.generated.cs

@@ -0,0 +1,9 @@
+using Godot;
+namespace ExternalModule {
+
+[ScriptPathAttribute("res://ExternalScript.cs")]
+partial class ExternalScript
+{
+}
+
+}

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Tests/TestData/Sources/ExternalScript.cs

@@ -0,0 +1,13 @@
+using Godot;
+
+namespace ExternalModule;
+
+public partial class ExternalScript : Node
+{
+    private int _health;
+
+    public override void _Ready()
+    {
+        _health = 100;
+    }
+}