makeDiskoTest: merge extraSystemConfig before prepareDiskoConfig

When extendModules is provided (from module.nix), use it to merge
extraSystemConfig with the base configuration BEFORE running
prepareDiskoConfig. This allows users to override disko settings
(partition sizes, LUKS keys, etc.) via disko.tests.extraConfig using
the proper NixOS module system rather than manual attribute merging.

The key change is that test-specific disko overrides are now merged
before device path transformation, so the overrides are included
when prepareDiskoConfig maps devices to QEMU virtio paths.

This makes PR nix-community#480 obsolete by using extendModules (enabled by PR nix-community#1094's
evalModules support for disko subtypes) instead of lib.recursiveUpdate.

Author: claude

lib/tests.nix

@@ -107,10 +107,25 @@ let
             importedDiskoConfig { inherit lib; }
           else
             importedDiskoConfig;
-        testConfigInstall = testLib.prepareDiskoConfig diskoConfigWithArgs (lib.tail testLib.devices);
+
+        # When extendModules is provided (from module.nix), we need to first merge
+        # extraSystemConfig to get the combined disko configuration, then run
+        # prepareDiskoConfig on the merged result. This allows users to override
+        # disko settings (like partition sizes, LUKS keys) via disko.tests.extraConfig.
+        mergedDiskoConfig =
+          if extendModules != null then
+            # Evaluate with extraSystemConfig to get merged disko settings
+            let
+              mergedEval = extendModules { modules = [ extraSystemConfig ]; };
+            in
+            builtins.removeAttrs mergedEval.config [ "_module" ]
+          else
+            diskoConfigWithArgs;
+
+        testConfigInstall = testLib.prepareDiskoConfig mergedDiskoConfig (lib.tail testLib.devices);
         # we need to shift the disks by one because the first disk is the /dev/vda of the test runner
         # so /dev/vdb becomes /dev/vda etc.
-        testConfigBooted = testLib.prepareDiskoConfig diskoConfigWithArgs testLib.devices;
+        testConfigBooted = testLib.prepareDiskoConfig mergedDiskoConfig testLib.devices;
 
         tsp-generator = pkgs.callPackage ../. { checked = true; };
         tsp-format = (tsp-generator._cliFormat testConfigInstall) pkgs;
@@ -121,6 +136,7 @@ let
         num-disks = builtins.length (lib.attrNames testConfigBooted.disko.devices.disk);
 
         # Module containing the base disko configuration for the installed system
+        # Only used when extendModules is NOT provided (standalone test mode)
         diskoModule =
           { config, ... }:
           {
@@ -147,6 +163,7 @@ let
           };
 
         # Module containing test-specific configuration for the VM environment
+        # This applies the QEMU device paths from prepareDiskoConfig
         testInstrumentationModule =
           { config, modulesPath, ... }:
           {
@@ -155,6 +172,10 @@ let
               (modulesPath + "/profiles/qemu-guest.nix")
             ];
 
+            # Apply the prepared disko config with QEMU device paths.
+            # We use mkForce because the device paths MUST be the QEMU virtio devices,
+            # but other disko settings (partition sizes, LUKS config, etc.) have already
+            # been merged via extraSystemConfig before prepareDiskoConfig was called.
             disko.devices = lib.mkForce testConfigBooted.disko.devices;
 
             # since we boot on a different machine, the efi payload needs to be portable
@@ -207,11 +228,13 @@ let
               system = pkgs.stdenv.hostPlatform.system;
             }).extendModules;
 
+        # When extendModules is provided, extraSystemConfig has already been merged
+        # into mergedDiskoConfig above, so we only add testInstrumentationModule here.
+        # When extendModules is NOT provided, we add both.
         installed-system-eval = baseExtendModules {
-          modules = [
-            testInstrumentationModule
-            extraSystemConfig
-          ];
+          modules =
+            [ testInstrumentationModule ]
+            ++ lib.optional (extendModules == null) extraSystemConfig;
         };
 
         installedTopLevel = installed-system-eval.config.system.build.toplevel;