auth: change OAuthHandler to take http Request and Response (#603)

Change OAuthHandler signature from func(context.Context,
OAuthHandlerArgs) to func(req *http.Request, res *http.Response).

- Remove OAuthHandlerArgs struct
- Update HTTPTransport to pass req and resp to handler
- Update tests to use new signature
- Handler can now call oauthex.GetProtectedResourceMetadataFromHeader
with proper validation against request URL

This change fixes an impedance mismatch between OAuthHandler and the
protected resource metadata functions of the oauthex package. The new
signature allows handlers to properly validate resource metadata against
the request URL, as required by RFC 9728.

Fixes #600

auth/client.go

@@ -8,26 +8,19 @@ package auth
 
 import (
 	"bytes"
-	"context"
 	"errors"
 	"io"
 	"net/http"
 	"sync"
 
-	"github.com/modelcontextprotocol/go-sdk/oauthex"
 	"golang.org/x/oauth2"
 )
 
 // An OAuthHandler conducts an OAuth flow and returns a [oauth2.TokenSource] if the authorization
 // is approved, or an error if not.
-type OAuthHandler func(context.Context, OAuthHandlerArgs) (oauth2.TokenSource, error)
-
-// OAuthHandlerArgs are arguments to an [OAuthHandler].
-type OAuthHandlerArgs struct {
-	// The URL to fetch protected resource metadata, extracted from the WWW-Authenticate header.
-	// Empty if not present or there was an error obtaining it.
-	ResourceMetadataURL string
-}
+// The handler receives the HTTP request and response that triggered the authentication flow.
+// To obtain the protected resource metadata, call [oauthex.GetProtectedResourceMetadataFromHeader].
+type OAuthHandler func(req *http.Request, res *http.Response) (oauth2.TokenSource, error)
 
 // HTTPTransport is an [http.RoundTripper] that follows the MCP
 // OAuth protocol when it encounters a 401 Unauthorized response.
@@ -112,10 +105,7 @@ func (t *HTTPTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 	// TODO: We hold the lock for the entire OAuth flow. This could be a long
 	// time. Is there a better way?
 	if _, ok := t.opts.Base.(*oauth2.Transport); !ok {
-		authHeaders := resp.Header[http.CanonicalHeaderKey("WWW-Authenticate")]
-		ts, err := t.handler(req.Context(), OAuthHandlerArgs{
-			ResourceMetadataURL: extractResourceMetadataURL(authHeaders),
-		})
+		ts, err := t.handler(req, resp)
 		if err != nil {
 			return nil, err
 		}
@@ -131,11 +121,3 @@ func (t *HTTPTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 
 	return t.opts.Base.RoundTrip(req)
 }
-
-func extractResourceMetadataURL(authHeaders []string) string {
-	cs, err := oauthex.ParseWWWAuthenticate(authHeaders)
-	if err != nil {
-		return ""
-	}
-	return oauthex.ResourceMetadataURL(cs)
-}