nextFreePort got out of port range on win (#19400)

on win:
```
  Error Trace:    D:/a/erigon/erigon/txnprovider/shutter/block_building_integration_test.go:335           
                                                                                                                    
  D:/a/erigon/erigon/txnprovider/shutter/block_building_integration_test.go:63                                        
              Error:          Received unexpected error:                                                          
                              failed to listen on any addresses: [listen tcp4 127.0.0.1:53528: bind: Only one         
  usage of each socket address (protocol/network address/port) is normally permitted.]                            
              Test:           TestShutterBlockBuilding      
```

Root cause: Port 53528 is in the Windows dynamic/ephemeral port range
(49152–65535). Windows uses this range to
assign outgoing TCP connections. After NextFreePort closes its probe
listener, the OS can immediately reassign that
port for an outbound connection — then when DialDecryptionKeysSender
tries to bind it (much later, after all the
  contract deployment setup), it's already taken.

Author: AskAlexSharov

common/freeport/free_port.go

@@ -27,8 +27,10 @@ import (
 )
 
 const (
-	maxPort = 65535
-	minPort = 1024
+	maxPort    = 49151 // stay below Windows ephemeral ports 49152-65535 (TCP connections assigned there)
+	minPort    = 1024
+	bucketSize = 2500
+	numBuckets = (maxPort - minPort) / bucketSize
 )
 
 var (
@@ -74,13 +76,12 @@ func nextPortNum(port int64) int64 {
 	if port == 0 { // init case
 		// generate a random starting point to avoid clashes
 		// if more than 1 "go test ./erigon " processes are run on the same machine at the same time
-		// for simplicity, assume there are 60,000 ports and each go test process needs 2500 ports
-		// 60,000/2500=24 buckets - randomly pick 1 of these buckets
+		// randomly pick 1 of numBuckets non-overlapping buckets
 		// note: this randomness is not needed for a single run of a "go test ./erigon" process
 		rnd := rand.New(rand.NewSource(time.Now().UnixNano()))
-		n := rnd.Intn(24)
-		return minPort + int64(n)*2500
-	} else if port == maxPort {
+		n := rnd.Intn(numBuckets)
+		return minPort + int64(n)*bucketSize
+	} else if port >= maxPort {
 		return minPort
 	} else {
 		return port + 1