Improve error handling with xerrors package

- Replace custom error handling with xerrors package
- Enhance error descriptions for better debugging
- Add text utilities for string processing
- Update error tests to use standard errors package
- Add String() method to GeminiError

🤖 Generated with [Claude Code](https://claude.ai/code)

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

gemini/errors_test.go

@@ -0,0 +1,36 @@
+package gemini
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+func TestErrGemini(t *testing.T) {
+	t.Parallel()
+	err := NewGeminiError(50, "50 server error")
+	if !errors.As(err, new(*GeminiError)) {
+		t.Errorf("TestErrGemini fail")
+	}
+}
+
+func TestErrGeminiWrapped(t *testing.T) {
+	t.Parallel()
+	err := NewGeminiError(50, "50 server error")
+	errWrapped := fmt.Errorf("%w wrapped", err)
+	if !errors.As(errWrapped, new(*GeminiError)) {
+		t.Errorf("TestErrGeminiWrapped fail")
+	}
+}
+
+func TestIsGeminiError(t *testing.T) {
+	t.Parallel()
+	err1 := NewGeminiError(50, "50 server error")
+	if !IsGeminiError(err1) {
+		t.Errorf("TestGeminiError fail #1")
+	}
+	wrappedErr1 := fmt.Errorf("wrapped %w", err1)
+	if !IsGeminiError(wrappedErr1) {
+		t.Errorf("TestGeminiError fail #2")
+	}
+}