all: stabilize client OAuth support (#861)

Also fix lint errors that were hidden because of the build tag.

auth/extauth/enterprise_handler.go

@@ -5,8 +5,6 @@
 // Package extauth provides OAuth handler implementations for MCP authorization extensions.
 // This package implements Enterprise Managed Authorization as defined in SEP-990.
 
-//go:build mcp_go_client_oauth
-
 package extauth
 
 import (

auth/extauth/enterprise_handler_test.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by an MIT-style
 // license that can be found in the LICENSE file.
 
-//go:build mcp_go_client_oauth
-
 package extauth
 
 import (

auth/extauth/oidc_login.go

@@ -6,8 +6,6 @@
 // as part of Enterprise Managed Authorization (SEP-990).
 // See https://openid.net/specs/openid-connect-core-1_0.html
 
-//go:build mcp_go_client_oauth
-
 package extauth
 
 import (
@@ -121,11 +119,11 @@ func initiateOIDCLogin(
 		return nil, nil, fmt.Errorf("RedirectURL is required")
 	}
 	if len(config.Scopes) == 0 {
-		return nil, nil, fmt.Errorf("Scopes is required (must include 'openid')")
+		return nil, nil, fmt.Errorf("at least one scope is required (must include 'openid')")
 	}
 
 	if !slices.Contains(config.Scopes, "openid") {
-		return nil, nil, fmt.Errorf("Scopes must include 'openid' for OIDC")
+		return nil, nil, fmt.Errorf("the 'openid' scope is required for OIDC")
 	}
 
 	if err := checkURLScheme(config.IssuerURL); err != nil {

auth/extauth/oidc_login_test.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by an MIT-style
 // license that can be found in the LICENSE file.
 
-//go:build mcp_go_client_oauth
-
 package extauth
 
 import (
@@ -161,7 +159,7 @@ func TestInitiateOIDCLogin(t *testing.T) {
 					c.Credentials = &oauthex.ClientCredentials{ClientID: "test"}
 					c.RedirectURL = "http://localhost:8080/callback"
 				},
-				expectErr: "Scopes is required",
+				expectErr: "at least one scope is required",
 			},
 		}
 		for _, tt := range tests {