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/models/markdown.go

@@ -1,7 +1,6 @@
 package models
 
 import (
-	"errors"
 	"fmt"
 	"strings"
 
@@ -109,8 +108,6 @@ func (s *Servers) toMarkdown(vpnProvider string) (formatted string, err error) {
 	return formatted, nil
 }
 
-var ErrMarkdownHeadersNotDefined = errors.New("markdown headers not defined")
-
 func getMarkdownHeaders(vpnProvider string) (headers []string, err error) {
 	switch vpnProvider {
 	case providers.Airvpn:
@@ -169,6 +166,6 @@ func getMarkdownHeaders(vpnProvider string) (headers []string, err error) {
 	case providers.Windscribe:
 		return []string{regionHeader, cityHeader, hostnameHeader, vpnHeader}, nil
 	default:
-		return nil, fmt.Errorf("%w: for %s", ErrMarkdownHeadersNotDefined, vpnProvider)
+		return nil, fmt.Errorf("markdown headers not defined: for %s", vpnProvider)
 	}
 }