refactor(grpc): restructure task handling and migrate to slog

Refactor the gRPC client and task handling logic to improve modularity,
error handling, and observability.

- Split `grpc/handler.go` into specialized handler files (login,
  classroom, exams, grades, schedule) to reduce complexity.
- Replace standard `log` with `log/slog` throughout the project for
  structured logging.
- Refactor `grpc/client.go` to use a thread-safe connection management
  pattern with `sync.RWMutex`.
- Remove the legacy `service` package and HTTP server implementation,
  transitioning the application to a pure gRPC runner mode.
- Clean up `config` and `pkg/errors` to remove unused utilities and
  simplify the API.
- Improve error reporting in parsers and HTTP clients by checking
  response status codes.

models/api.go

@@ -1,38 +0,0 @@
-package models
-
-// APIRequest API请求结构
-type APIRequest struct {
-	Model          string    `json:"model"`
-	Messages       []Message `json:"messages"`
-	MaxTokens      int       `json:"max_tokens"`
-	EnableThinking *bool     `json:"enable_thinking,omitempty"`
-	ThinkingBudget *int      `json:"thinking_budget,omitempty"`
-}
-
-// Message 消息结构
-type Message struct {
-	Role    string `json:"role"`
-	Content any    `json:"content"`
-}
-
-// ContentItem 内容项
-type ContentItem struct {
-	Type     string    `json:"type,omitempty"`
-	Text     string    `json:"text,omitempty"`
-	ImageURL *ImageURL `json:"image_url,omitempty"`
-}
-
-// ImageURL 图片URL
-type ImageURL struct {
-	URL string `json:"url"`
-}
-
-// APIResponse API响应结构
-type APIResponse struct {
-	Choices []Choice `json:"choices"`
-}
-
-// Choice 选择
-type Choice struct {
-	Message Message `json:"message"`
-}