feat(sdk): add Go generator

Implements Go SDK generator with two-level fallback:
1. Native oapi-codegen if installed (generates types and client)
2. Docker openapitools/openapi-generator-cli as fallback

Native generation also creates a minimal go.mod file.
Includes tests following _Good convention for Available and Generate.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pkg/sdk/generators/go.go

@@ -0,0 +1,83 @@
+package generators
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+)
+
+// GoGenerator generates Go SDKs from OpenAPI specs.
+type GoGenerator struct{}
+
+// NewGoGenerator creates a new Go generator.
+func NewGoGenerator() *GoGenerator {
+	return &GoGenerator{}
+}
+
+// Language returns the generator's target language identifier.
+func (g *GoGenerator) Language() string {
+	return "go"
+}
+
+// Available checks if generator dependencies are installed.
+func (g *GoGenerator) Available() bool {
+	_, err := exec.LookPath("oapi-codegen")
+	return err == nil
+}
+
+// Install returns instructions for installing the generator.
+func (g *GoGenerator) Install() string {
+	return "go install github.com/oapi-codegen/oapi-codegen/v2/cmd/oapi-codegen@latest"
+}
+
+// Generate creates SDK from OpenAPI spec.
+func (g *GoGenerator) Generate(ctx context.Context, opts Options) error {
+	if err := os.MkdirAll(opts.OutputDir, 0755); err != nil {
+		return fmt.Errorf("go.Generate: failed to create output dir: %w", err)
+	}
+
+	if g.Available() {
+		return g.generateNative(ctx, opts)
+	}
+	return g.generateDocker(ctx, opts)
+}
+
+func (g *GoGenerator) generateNative(ctx context.Context, opts Options) error {
+	outputFile := filepath.Join(opts.OutputDir, "client.go")
+
+	cmd := exec.CommandContext(ctx, "oapi-codegen",
+		"-package", opts.PackageName,
+		"-generate", "types,client",
+		"-o", outputFile,
+		opts.SpecPath,
+	)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+
+	if err := cmd.Run(); err != nil {
+		return fmt.Errorf("go.generateNative: %w", err)
+	}
+
+	goMod := fmt.Sprintf("module %s\n\ngo 1.21\n", opts.PackageName)
+	return os.WriteFile(filepath.Join(opts.OutputDir, "go.mod"), []byte(goMod), 0644)
+}
+
+func (g *GoGenerator) generateDocker(ctx context.Context, opts Options) error {
+	specDir := filepath.Dir(opts.SpecPath)
+	specName := filepath.Base(opts.SpecPath)
+
+	cmd := exec.CommandContext(ctx, "docker", "run", "--rm",
+		"-v", specDir+":/spec",
+		"-v", opts.OutputDir+":/out",
+		"openapitools/openapi-generator-cli", "generate",
+		"-i", "/spec/"+specName,
+		"-g", "go",
+		"-o", "/out",
+		"--additional-properties=packageName="+opts.PackageName,
+	)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	return cmd.Run()
+}

pkg/sdk/generators/go_test.go

@@ -0,0 +1,58 @@
+package generators
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+func TestGoGenerator_Good_Available(t *testing.T) {
+	g := NewGoGenerator()
+
+	// These should not panic
+	lang := g.Language()
+	if lang != "go" {
+		t.Errorf("expected language 'go', got '%s'", lang)
+	}
+
+	_ = g.Available()
+
+	install := g.Install()
+	if install == "" {
+		t.Error("expected non-empty install instructions")
+	}
+}
+
+func TestGoGenerator_Good_Generate(t *testing.T) {
+	g := NewGoGenerator()
+	if !g.Available() && !dockerAvailable() {
+		t.Skip("no Go generator available (neither native nor docker)")
+	}
+
+	// Create temp directories
+	tmpDir := t.TempDir()
+	specPath := createTestSpec(t, tmpDir)
+	outputDir := filepath.Join(tmpDir, "output")
+
+	opts := Options{
+		SpecPath:    specPath,
+		OutputDir:   outputDir,
+		PackageName: "testclient",
+		Version:     "1.0.0",
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Minute)
+	defer cancel()
+
+	err := g.Generate(ctx, opts)
+	if err != nil {
+		t.Fatalf("Generate failed: %v", err)
+	}
+
+	// Verify output directory was created
+	if _, err := os.Stat(outputDir); os.IsNotExist(err) {
+		t.Error("output directory was not created")
+	}
+}