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.go

@@ -1,9 +1,8 @@
 package gemini
 
 import (
+	"errors"
 	"fmt"
-
-	"github.com/antanst/go_errors"
 )
 
 // GeminiError is used to represent
@@ -20,6 +19,10 @@ func (e *GeminiError) Error() string {
 	return fmt.Sprintf("gemini error: code %d %s", e.Code, e.Msg)
 }
 
+func (e *GeminiError) String() string {
+	return e.Error()
+}
+
 // NewGeminiError creates a new GeminiError based on the status code and header.
 // Status codes are based on the Gemini protocol specification:
 // - 1x: Input required
@@ -57,5 +60,5 @@ func IsGeminiError(err error) bool {
 		return false
 	}
 	var asError *GeminiError
-	return go_errors.As(err, &asError)
+	return errors.As(err, &asError)
 }

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")
+	}
+}