fix(api): improve error message for file not found in GetAgentSkillFile (#500)

Previously, when a skill file was not found, the API returned:
- HTTP 404 status code
- Generic "database error" message (from serializer.DBErr)

This was confusing because:
1. The user didn't know if it was a database issue or file missing
2. "database error" sounds like a server-side problem

Now it returns:
- HTTP 404 status code
- Clear "file not found" message

This helps users understand that the requested file simply doesn't exist
in the skill, rather than encountering a database error.

Author: mbt1909432

src/server/api/go/internal/modules/handler/agent_skills.go

@@ -272,7 +272,7 @@ func (h *AgentSkillsHandler) GetAgentSkillFile(c *gin.Context) {
 	output, err := h.svc.GetFile(c.Request.Context(), project.ID, id, filePath, expire)
 	if err != nil {
 		if strings.Contains(err.Error(), "not found") {
-			c.JSON(http.StatusNotFound, serializer.DBErr("", err))
+			c.JSON(http.StatusNotFound, serializer.Err(http.StatusNotFound, "file not found", err))
 		} else {
 			c.JSON(http.StatusInternalServerError, serializer.DBErr("", err))
 		}