refactor: improve error messages in M2M config loading for clarity

Jira Ticket: https://linuxfoundation.atlassian.net/browse/LFXV2-501

Assisted by [Claude Code](https://claude.ai/code)

Signed-off-by: Mauricio Zanetti Salomao <mauriciozanetti86@gmail.com>

Author: mauriciozanettisalomao

internal/infrastructure/auth0/token.go

@@ -142,23 +142,23 @@ func (tm *TokenManager) GetTokenInfo() (*TokenInfo, error) {
 func loadM2MConfigFromEnv(ctx context.Context, config Config) (m2mConfig, error) {
 	clientID := os.Getenv(constants.Auth0M2MClientIDEnvKey)
 	if clientID == "" {
-		return m2mConfig{}, errors.NewUnexpected("AUTH0_M2M_CLIENT_ID is required")
+		return m2mConfig{}, errors.NewUnexpected(constants.Auth0M2MClientIDEnvKey + " is required")
 	}
 
 	audience := os.Getenv(constants.Auth0AudienceEnvKey)
 	if audience == "" {
-		return m2mConfig{}, errors.NewUnexpected("AUTH0_AUDIENCE is required")
+		return m2mConfig{}, errors.NewUnexpected(constants.Auth0AudienceEnvKey + " is required")
 	}
 
 	// private key is base64 encoded
 	privateKey := os.Getenv(constants.Auth0M2MPrivateBase64KeyEnvKey)
 	if privateKey == "" {
-		return m2mConfig{}, errors.NewUnexpected("AUTH0_M2M_PRIVATE_BASE64_KEY is required")
+		return m2mConfig{}, errors.NewUnexpected(constants.Auth0M2MPrivateBase64KeyEnvKey + " is required")
 	}
 
 	decoded, err := base64.StdEncoding.DecodeString(privateKey)
 	if err != nil {
-		return m2mConfig{}, errors.NewUnexpected("failed to base64-decode AUTH0_M2M_PRIVATE_BASE64_KEY", err)
+		return m2mConfig{}, errors.NewUnexpected("failed to base64-decode "+constants.Auth0M2MPrivateBase64KeyEnvKey, err)
 	}
 	privateKey = string(decoded)
 	//
@@ -222,12 +222,12 @@ func NewM2MTokenManager(ctx context.Context, config Config) (*TokenManager, erro
 func NewProfileClientAuthConfig(ctx context.Context, domain string) (*authentication.Authentication, error) {
 	clientID := os.Getenv(constants.Auth0LFXProfileClientIDEnvKey)
 	if clientID == "" {
-		return nil, errors.NewUnexpected("AUTH0_LFX_PROFILE_CLIENT_ID is required for email linking flow")
+		return nil, errors.NewUnexpected(constants.Auth0LFXProfileClientIDEnvKey + " is required for email linking flow")
 	}
 
 	clientSecret := os.Getenv(constants.Auth0LFXProfileClientSecretEnvKey)
 	if clientSecret == "" {
-		return nil, errors.NewUnexpected("AUTH0_LFX_PROFILE_CLIENT_SECRET is required for email linking flow")
+		return nil, errors.NewUnexpected(constants.Auth0LFXProfileClientSecretEnvKey + " is required for email linking flow")
 	}
 
 	// Create Auth0 authentication client with client secret