Fix tests: use StartTestCustomServer (no Content-Length) to match real bug scenario

Remove StartTestNoRangeServer and its tests since the real bug is about
servers that don't return Content-Length at all (like GitHub's zip downloads),
not servers with Content-Length but no Range support.

The chunked tests (TestFetcher_DownloadChunkedContinue and
TestFetcher_DownloadChunkedNotExceedSize) correctly simulate the real-world
bug using StartTestCustomServer which serves files without Content-Length.

Co-authored-by: monkeyWie <[email protected]>

internal/test/httptest.go

@@ -636,25 +636,6 @@ func StartTestRangeBugServer() net.Listener {
 	})
 }
 
-// StartTestNoRangeServer starts a server that returns Content-Length but does NOT support Range requests.
-// This simulates servers where file size is known but range downloads are not supported.
-func StartTestNoRangeServer() net.Listener {
-	return startTestServer(func(sl *shutdownListener) http.Handler {
-		mux := http.NewServeMux()
-		mux.HandleFunc("/"+BuildName, func(writer http.ResponseWriter, request *http.Request) {
-			file, err := os.Open(BuildFile)
-			if err != nil {
-				panic(err)
-			}
-			defer file.Close()
-			writer.Header().Set("Content-Length", fmt.Sprintf("%d", BuildSize))
-			writer.WriteHeader(200)
-			io.Copy(writer, file)
-		})
-		return mux
-	})
-}
-
 func rangeFileHandle(writer http.ResponseWriter, request *http.Request, modifyEnd func(end int64) int64, iocpN func(file *os.File, n int64)) {
 	r := request.Header.Get("Range")