Reduce handler request boilerplate

- Add session.Attach to reuse per-request sessions across middleware and ghome handlers
- Make Session finalize/abort paths idempotent to avoid duplicate commit or rollback work
- Move request_data parsing helper to internal/utils
- Expand ParseRequestData usage across api, download, unit, live, multiunit, and subscenario handlers
- Extract download package metadata into internal/handler/download/types.go
- Return explicit errors from selected session lookup helpers and update callers
- Make api action handlers return errors directly instead of calling ss.CheckErr in nested layers

Signed-off-by: Sean Du <do4suki@gmail.com>

internal/handler/ghome/basic/getcode.go

@@ -10,7 +10,7 @@ import (
 )
 
 func getCode(ctx *gin.Context) {
-	ss := session.New(ctx)
+	ss := session.Attach(ctx)
 	defer ss.Finalize()
 
 	codeArray := `{"codeArray":[{"btntext":"好的","code":"-10264022","msg_from":2,"text":"","title":"短信验证码被阻止","type":1},{"btntext":"","code":"-10869623","msg_from":2,"text":"","title":"网络连接失败，无法一键登录","type":2},{"btntext":"","code":"10298300","msg_from":2,"text":"","title":"","type":3},{"btntext":"","code":"10298311","msg_from":2,"text":"","title":"","type":3},{"btntext":"","code":"10298312","msg_from":2,"text":"","title":"","type":3},{"btntext":"","code":"10298313","msg_from":2,"text":"","title":"","type":1},{"btntext":"","code":"10298321","msg_from":2,"text":"","title":"","type":3},{"btntext":"","code":"10298322","msg_from":2,"text":"","title":"","type":3}],"codeVersion":"1.0.5"}`