fix: delete XDG_CONFIG_HOME when test env opts out of setting it

When setupTestEnv({ setXdgConfig: false }) is called, explicitly delete
XDG_CONFIG_HOME so getConfigDir() falls back to $HOME/.config. Without
this, a pre-existing XDG_CONFIG_HOME (e.g. from CI) causes diagram-service
tests to look in a different directory than where test diagrams are created.

Author: veelenga

test/helpers/env-helpers.ts

@@ -32,6 +32,8 @@ export async function setupTestEnv(opts: SetupOptions = {}): Promise<string> {
     const tempConfigDir = join(tempHome, ".config");
     process.env.XDG_CONFIG_HOME = tempConfigDir;
     await mkdir(tempConfigDir, { recursive: true });
+  } else {
+    delete process.env.XDG_CONFIG_HOME;
   }
 
   return tempHome;