chore: do not use sentinel errors when unneeded

- main reason being it's a burden to always define sentinel errors at global scope, wrap them with `%w` instead of using a string directly
- only use sentinel errors when it has to be checked using `errors.Is`
- replace all usage of these sentinel errors in `fmt.Errorf` with direct strings that were in the sentinel error
- exclude the sentinel error definition requirement from .golangci.yml
- update unit tests to use ContainersError instead of ErrorIs so it stays as a "not a change detector test" without requiring a sentinel error

internal/natpmp/portmapping.go

@@ -3,17 +3,11 @@ package natpmp
 import (
 	"context"
 	"encoding/binary"
-	"errors"
 	"fmt"
 	"net/netip"
 	"time"
 )
 
-var (
-	ErrNetworkProtocolUnknown = errors.New("network protocol is unknown")
-	ErrLifetimeTooLong        = errors.New("lifetime is too long")
-)
-
 // Add or delete a port mapping. To delete a mapping, set both the
 // requestedExternalPort and lifetime to 0.
 // See https://www.ietf.org/rfc/rfc6886.html#section-3.3
@@ -26,8 +20,9 @@ func (c *Client) AddPortMapping(ctx context.Context, gateway netip.Addr,
 	lifetimeSecondsFloat := lifetime.Seconds()
 	const maxLifetimeSeconds = uint64(^uint32(0))
 	if uint64(lifetimeSecondsFloat) > maxLifetimeSeconds {
-		return 0, 0, 0, 0, fmt.Errorf("%w: %d seconds must at most %d seconds",
-			ErrLifetimeTooLong, uint64(lifetimeSecondsFloat), maxLifetimeSeconds)
+		return 0, 0, 0, 0, fmt.Errorf("lifetime is too long: "+
+			"%d seconds must at most %d seconds",
+			uint64(lifetimeSecondsFloat), maxLifetimeSeconds)
 	}
 	const messageSize = 12
 	message := make([]byte, messageSize)
@@ -38,7 +33,7 @@ func (c *Client) AddPortMapping(ctx context.Context, gateway netip.Addr,
 	case "tcp":
 		message[1] = 2 // operationCode 2
 	default:
-		return 0, 0, 0, 0, fmt.Errorf("%w: %s", ErrNetworkProtocolUnknown, protocol)
+		return 0, 0, 0, 0, fmt.Errorf("network protocol is unknown: %s", protocol)
 	}
 	// [2:3] are reserved.
 	binary.BigEndian.PutUint16(message[4:6], internalPort)