fix(config): read default config if one does not exist

Right now, aecpli returns an error if a config file doesn't exist:

```
2025/05/03 11:22:14 ERROR Config file does not exist file=$HOME/.config/aepcli/config.toml
unable to read config: stat $HOME/.config/aepcli/config.toml: no such file or directory
```

The CLI should not fail if a config file does not exist.

Author: toumorokoshi

internal/config/config.go

@@ -25,8 +25,8 @@ type API struct {
 func ReadConfigFromFile(file string) (*Config, error) {
 	// Check if file exists first
 	if _, err := os.Stat(file); os.IsNotExist(err) {
-		slog.Error("Config file does not exist", "file", file)
-		return nil, err
+		slog.Warn("Config file does not exist, using default configuration", "file", file)
+		return &Config{APIs: make(map[string]API)}, nil
 	}
 
 	var c Config

internal/config/config_test.go

@@ -1,8 +1,6 @@
 package config
 
 import (
-	"errors"
-	"os"
 	"path/filepath"
 	"testing"
 
@@ -14,10 +12,11 @@ func TestConfigEndToEnd(t *testing.T) {
 	tmpDir := t.TempDir()
 	testFile := filepath.Join(tmpDir, "test_config.toml")
 
-	// First read should fail since file doesn't exist
-	_, err := ReadConfigFromFile(testFile)
-	assert.Error(t, err)
-	assert.True(t, errors.Is(err, os.ErrNotExist))
+	// First read should return a default configuration since file doesn't exist
+	cfg, err := ReadConfigFromFile(testFile)
+	assert.NoError(t, err)
+	assert.NotNil(t, cfg)
+	assert.Empty(t, cfg.APIs)
 
 	// Create test API config
 	testAPI := API{
@@ -33,7 +32,7 @@ func TestConfigEndToEnd(t *testing.T) {
 	assert.NoError(t, err)
 
 	// Read config back and verify contents
-	cfg, err := ReadConfigFromFile(testFile)
+	cfg, err = ReadConfigFromFile(testFile)
 	assert.NoError(t, err)
 	assert.NotNil(t, cfg)
 	assert.Len(t, cfg.APIs, 1)
@@ -61,3 +60,17 @@ func TestWriteAPIWithEmptyName(t *testing.T) {
 	assert.Error(t, err)
 	assert.Equal(t, "api name cannot be empty", err.Error())
 }
+
+func TestReadConfigFromFile_NoFile(t *testing.T) {
+	// Create a temporary directory
+	tmpDir := t.TempDir()
+	testFile := filepath.Join(tmpDir, "nonexistent_config.toml")
+
+	// Attempt to read the config file
+	cfg, err := ReadConfigFromFile(testFile)
+
+	// Verify no error is returned and a default config is provided
+	assert.NoError(t, err)
+	assert.NotNil(t, cfg)
+	assert.Empty(t, cfg.APIs)
+}