fix: add command executor interface for better testing

Introduce the CommandExecutor interface to abstract command execution, 
allowing for easier mocking in tests. Implement DefaultCommandExecutor 
to use the os/exec package for executing commands. Update the 
GenerateCosmoRouterConfig function to utilize the new 
GenerateCosmoRouterConfigWithExecutor function that accepts a command 
executor parameter. Add a MockCommandExecutor for simulating command 
execution in unit tests with realistic behavior based on input YAML 
files. This enhances test coverage and simplifies error handling.

graph/cosmo.go

@@ -11,9 +11,30 @@ import (
 	"gitlab.com/unboundsoftware/schemas/graph/model"
 )
 
+// CommandExecutor is an interface for executing external commands
+// This allows for mocking in tests
+type CommandExecutor interface {
+	Execute(name string, args ...string) ([]byte, error)
+}
+
+// DefaultCommandExecutor implements CommandExecutor using os/exec
+type DefaultCommandExecutor struct{}
+
+// Execute runs a command and returns its combined output
+func (e *DefaultCommandExecutor) Execute(name string, args ...string) ([]byte, error) {
+	cmd := exec.Command(name, args...)
+	return cmd.CombinedOutput()
+}
+
 // GenerateCosmoRouterConfig generates a Cosmo Router execution config from subgraphs
 // using the official wgc CLI tool via npx
 func GenerateCosmoRouterConfig(subGraphs []*model.SubGraph) (string, error) {
+	return GenerateCosmoRouterConfigWithExecutor(subGraphs, &DefaultCommandExecutor{})
+}
+
+// GenerateCosmoRouterConfigWithExecutor generates a Cosmo Router execution config from subgraphs
+// using the provided command executor (useful for testing)
+func GenerateCosmoRouterConfigWithExecutor(subGraphs []*model.SubGraph, executor CommandExecutor) (string, error) {
 	if len(subGraphs) == 0 {
 		return "", fmt.Errorf("no subgraphs provided")
 	}
@@ -85,13 +106,11 @@ func GenerateCosmoRouterConfig(subGraphs []*model.SubGraph) (string, error) {
 	// Execute wgc router compose
 	// wgc is installed globally in the Docker image
 	outputFile := filepath.Join(tmpDir, "config.json")
-	cmd := exec.Command("wgc", "router", "compose",
+	output, err := executor.Execute("wgc", "router", "compose",
 		"--input", inputFile,
 		"--out", outputFile,
 		"--suppress-warnings",
 	)
-
-	output, err := cmd.CombinedOutput()
 	if err != nil {
 		return "", fmt.Errorf("wgc router compose failed: %w\nOutput: %s", err, string(output))
 	}