Build request data through RequestData struct instead of fasthttp directly

Refactor request generators to populate a script.RequestData intermediate struct, then apply it to fasthttp.Request in one step. This eliminates the round-trip conversion (fasthttp → RequestData → fasthttp) when scripts are enabled. Remove the URL field from RequestData and the now-unused fasthttp conversion functions from chain.go.

internal/script/script.go

@@ -17,7 +17,6 @@ import (
 // Headers, Params, and Cookies use []string values to support multiple values per key.
 type RequestData struct {
 	Method  string              `json:"method"`
-	URL     string              `json:"url"`
 	Path    string              `json:"path"`
 	Headers map[string][]string `json:"headers"`
 	Params  map[string][]string `json:"params"`