feat(grpc): add support for fetching grades and exams

Implement new gRPC task handlers for retrieving student grades and exam schedules. This includes adding new parser logic for processing score and exam data, and updating the client capabilities to advertise support for these new task types.

- Add `getGrades` and `getExams` handlers in `grpc/handler.go`
- Add `parser/encoding.go`, `parser/exam.go`, and `parser/score.go` for data parsing
- Update `grpc/client.go` to include `TASK_TYPE_GET_GRADES` and `TASK_TYPE_GET_EXAMS` in capabilities

parser/encoding.go

@@ -0,0 +1,74 @@
+package parser
+
+import (
+	"bytes"
+	"io"
+	"net/http"
+	"strings"
+
+	"golang.org/x/text/encoding/simplifiedchinese"
+	"golang.org/x/text/transform"
+)
+
+func decodeResponseBody(resp *http.Response) (string, error) {
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return "", err
+	}
+	return decodeBody(body, resp.Header.Get("Content-Type")), nil
+}
+
+func decodeBody(body []byte, contentType string) string {
+	lowerCT := strings.ToLower(contentType)
+	if strings.Contains(lowerCT, "charset=gb") || strings.Contains(lowerCT, "charset=gb2312") || strings.Contains(lowerCT, "charset=gbk") {
+		decoded, err := decodeGBK(body)
+		if err == nil {
+			return decoded
+		}
+	}
+
+	if isValidUTF8(body) {
+		return string(body)
+	}
+
+	decoded, err := decodeGBK(body)
+	if err == nil {
+		return decoded
+	}
+	return string(body)
+}
+
+func decodeGBK(data []byte) (string, error) {
+	reader := transform.NewReader(bytes.NewReader(data), simplifiedchinese.GBK.NewDecoder())
+	decoded, err := io.ReadAll(reader)
+	if err != nil {
+		return "", err
+	}
+	return string(decoded), nil
+}
+
+func isValidUTF8(data []byte) bool {
+	for i := 0; i < len(data); i++ {
+		if data[i]&0x80 != 0 {
+			if data[i]&0xE0 == 0xC0 {
+				if i+1 >= len(data) || data[i+1]&0xC0 != 0x80 {
+					return false
+				}
+				i++
+			} else if data[i]&0xF0 == 0xE0 {
+				if i+2 >= len(data) || data[i+1]&0xC0 != 0x80 || data[i+2]&0xC0 != 0x80 {
+					return false
+				}
+				i += 2
+			} else if data[i]&0xF8 == 0xF0 {
+				if i+3 >= len(data) || data[i+1]&0xC0 != 0x80 || data[i+2]&0xC0 != 0x80 || data[i+3]&0xC0 != 0x80 {
+					return false
+				}
+				i += 3
+			} else {
+				return false
+			}
+		}
+	}
+	return true
+}