build(nix): add bun, pnpm, and typescript-go to devShell

Add Node.js tooling required to run the MCP mock server for testing:
- bun: Fast JavaScript runtime used to run the mock server
- pnpm: Package manager for installing Node dependencies
- typescript-go: TypeScript compiler

Author: ryoppippi

flake.nix

@@ -98,6 +98,11 @@
 
               # security
               gitleaks
+
+              # Node.js for MCP mock server
+              bun
+              pnpm
+              typescript-go
             ];
 
             shellHook = ''