Move form_builder into internal pkg. (#311)

* Move form_uilder into internal pkg.

* Fix import of audio.go

* Reorganize.

* Fix import.

* Fix

---------

Co-authored-by: JoyShi <joy.shi@sap.com>

internal/form_builder_test.go

@@ -0,0 +1,54 @@
+package openai //nolint:testpackage // testing private field
+
+import (
+	"github.com/sashabaranov/go-openai/internal/test"
+	"github.com/sashabaranov/go-openai/internal/test/checks"
+
+	"bytes"
+	"errors"
+	"os"
+	"testing"
+)
+
+type failingWriter struct {
+}
+
+var errMockFailingWriterError = errors.New("mock writer failed")
+
+func (*failingWriter) Write([]byte) (int, error) {
+	return 0, errMockFailingWriterError
+}
+
+func TestFormBuilderWithFailingWriter(t *testing.T) {
+	dir, cleanup := test.CreateTestDirectory(t)
+	defer cleanup()
+
+	file, err := os.CreateTemp(dir, "")
+	if err != nil {
+		t.Errorf("Error creating tmp file: %v", err)
+	}
+	defer file.Close()
+	defer os.Remove(file.Name())
+
+	builder := NewFormBuilder(&failingWriter{})
+	err = builder.CreateFormFile("file", file)
+	checks.ErrorIs(t, err, errMockFailingWriterError, "formbuilder should return error if writer fails")
+}
+
+func TestFormBuilderWithClosedFile(t *testing.T) {
+	dir, cleanup := test.CreateTestDirectory(t)
+	defer cleanup()
+
+	file, err := os.CreateTemp(dir, "")
+	if err != nil {
+		t.Errorf("Error creating tmp file: %v", err)
+	}
+	file.Close()
+	defer os.Remove(file.Name())
+
+	body := &bytes.Buffer{}
+	builder := NewFormBuilder(body)
+	err = builder.CreateFormFile("file", file)
+	checks.HasError(t, err, "formbuilder should return error if file is closed")
+	checks.ErrorIs(t, err, os.ErrClosed, "formbuilder should return error if file is closed")
+}