vscode: enable defining mcp.json separate from settings.json (#7441)

VS Code 1.102 separates MCP configuration from `settings.json` to a profile-specific `mcp.json`. VS Code automatically performs this separation if MCP configuration is detected inside `settings.json` which conflicts with the immutability of the settings.json that home-manager supplies.

tests/modules/programs/vscode/default.nix

@@ -1,6 +1,7 @@
 {
   vscode-keybindings = ./keybindings.nix;
   vscode-tasks = ./tasks.nix;
+  vscode-mcp = ./mcp.nix;
   vscode-update-checks = ./update-checks.nix;
   vscode-snippets = ./snippets.nix;
 }

tests/modules/programs/vscode/mcp.nix

@@ -0,0 +1,71 @@
+{ pkgs, lib, ... }:
+
+let
+
+  mcpFilePath =
+    name:
+    if pkgs.stdenv.hostPlatform.isDarwin then
+      "Library/Application Support/Code/User/${
+        lib.optionalString (name != "default") "profiles/${name}/"
+      }mcp.json"
+    else
+      ".config/Code/User/${lib.optionalString (name != "default") "profiles/${name}/"}mcp.json";
+
+  content = ''
+    {
+      "servers": {
+        "Github": {
+          "url": "https://api.githubcopilot.com/mcp/"
+        }
+      }
+    }
+  '';
+
+  mcp = {
+    servers = {
+      Github = {
+        url = "https://api.githubcopilot.com/mcp/";
+      };
+    };
+  };
+
+  customMcpPath = pkgs.writeText "custom.json" content;
+
+  expectedMcp = pkgs.writeText "mcp-expected.json" ''
+    {
+      "servers": {
+        "Github": {
+          "url": "https://api.githubcopilot.com/mcp/"
+        }
+      }
+    }
+  '';
+
+  expectedCustomMcp = pkgs.writeText "custom-expected.json" content;
+
+in
+{
+  programs.vscode = {
+    enable = true;
+    package = pkgs.writeScriptBin "vscode" "" // {
+      pname = "vscode";
+      version = "1.75.0";
+    };
+    profiles = {
+      default.userMcp = mcp;
+      test.userMcp = mcp;
+      custom.userMcp = customMcpPath;
+    };
+  };
+
+  nmt.script = ''
+    assertFileExists "home-files/${mcpFilePath "default"}"
+    assertFileContent "home-files/${mcpFilePath "default"}" "${expectedMcp}"
+
+    assertFileExists "home-files/${mcpFilePath "test"}"
+    assertFileContent "home-files/${mcpFilePath "test"}" "${expectedMcp}"
+
+    assertFileExists "home-files/${mcpFilePath "custom"}"
+    assertFileContent "home-files/${mcpFilePath "custom"}" "${expectedCustomMcp}"
+  '';
+}