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/utils/request.go

@@ -0,0 +1,11 @@
+package utils
+
+import (
+	"encoding/json"
+
+	"github.com/gin-gonic/gin"
+)
+
+func ParseRequestData(ctx *gin.Context, dst any) error {
+	return json.Unmarshal([]byte(ctx.MustGet("request_data").(string)), dst)
+}