Add call graph fixtures for various languages and scenarios

- Introduced `all-edge-reasons.json` to test edge resolution reasons in .NET.
- Added `all-visibility-levels.json` to validate method visibility levels in .NET.
- Created `dotnet-aspnetcore-minimal.json` for a minimal ASP.NET Core application.
- Included `go-gin-api.json` for a Go Gin API application structure.
- Added `java-spring-boot.json` for the Spring PetClinic application in Java.
- Introduced `legacy-no-schema.json` for legacy application structure without schema.
- Created `node-express-api.json` for an Express.js API application structure.

tests/reachability/fixtures/callgraph-schema-v1/legacy-no-schema.json

@@ -0,0 +1,47 @@
+{
+  "id": "cg-legacy-001",
+  "languageString": "csharp",
+  "component": "LegacyApp",
+  "version": "0.9.0",
+  "ingestedAt": "2024-06-15T08:00:00Z",
+  "graphHash": "sha256:legacy123",
+  "nodes": [
+    {
+      "id": "l001",
+      "name": "Main",
+      "kind": "method",
+      "namespace": "LegacyApp"
+    },
+    {
+      "id": "l002",
+      "name": "ProcessData",
+      "kind": "method",
+      "namespace": "LegacyApp.Controllers"
+    },
+    {
+      "id": "l003",
+      "name": "ValidateInput",
+      "kind": "method",
+      "namespace": "LegacyApp.Internal"
+    }
+  ],
+  "edges": [
+    {
+      "sourceId": "l001",
+      "targetId": "l002",
+      "type": "call"
+    },
+    {
+      "sourceId": "l002",
+      "targetId": "l003",
+      "type": "call"
+    }
+  ],
+  "roots": [
+    {
+      "id": "l001",
+      "phase": "startup",
+      "source": "convention"
+    }
+  ]
+}