all: client side OAuth support (#785)

This PR introduces client-side OAuth support.

# Overall design philosophy

The main new interface introduced to support client-side OAuth is the
`OAuthHandler`. Contrary to its counter-parts in some other SDKs, it is
intentionally small and generic so that it can support most of the OAuth
authorization flow variants. The main `mcp` package purposefully
delegates almost all auth handling to `OAuthHandler` implementations.
This way, the `mcp` package continues to be largely OAuth agnostic.

The `auth` package comes with a default `OAuthHandler` implementation –
`AuthorizationCodeHandler`, which implements the authorization flow
for the `authorization_code` grant type, as defined in the [MCP
specification](https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization).
It is expected that the `OAuthHandler` implementation granularity will
be roughly per grant type, as this parameter influences the
authorization flow the most.

# Detailed design

## `OAuthHandler` (auth/client.go)

The main interface for the integration consists of two methods:

```go
type OAuthHandler interface {
	// TokenSource returns a token source to be used for outgoing requests.
	// Returned token source might be nil. In that case, the transport will not
	// add any authorization headers to the request.
	TokenSource(context.Context) (oauth2.TokenSource, error)

	// Authorize is called when an HTTP request results in an error that may
	// be addressed by the authorization flow (currently 401 Unauthorized and 403 Forbidden).
	// It is responsible for performing the OAuth flow to obtain an access token.
	// The arguments are the request that failed and the response that was received for it.
	// The headers of the request are available, but the body will have already been consumed
	// when Authorize is called.
	// If the returned error is nil, TokenSource is expected to return a non-nil token source.
	// After a successful call to Authorize, the HTTP request will be retried by the transport.
	// The function is responsible for closing the response body.
	Authorize(context.Context, *http.Request, *http.Response) error
}
```

`StreamableClientTransport` accepts this interface as a field and uses
the `TokenSource` method to add an `Authorization: Bearer` header to
outgoing requests to the MCP server. If the request fails with `401:
Unauthorized` or `403: Forbidden`, the transport calls `Authorize` to
perform the authorization flow. If `Authorize` returns a non-nil error,
the request is retried once. `Authorize` errors do not result in
termination of the client session (unless they happen during
connection initialization), as some OAuth flows are multi-legged and it should be
acceptable to retry for example when the authorization grant was
received.

*Note:* There was already an `OAuthHandler` type defined as part of a
previous iteration of OAuth support. It was renamed to
`OAuthHandlerLegacy`. This is the only backwards incompatible API change
in the experimental client `auth` part (protected by the
`mcp_go_client_oauth` Go build tag). The cost of replacing previous
usages is deemed worthwhile to maintain clear naming, under the
assumption that the experimental client `auth` part was not widely used.

## `AuthorizationCodeHandler` (auth/authorization_code.go)

This is the `OAuthHandler` implementation that fulfills the [MCP
specification](https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization).
In particular, in supports:

- Protected Resource Metadata discovery and fetching
- Authorization Server Metadata discovery and fetching
- Client registration
  - OpenID Connect metadata document
  - Pre-registration
  - Dynamic Client Registration
- PKCE
- `client_secret_post`, `client_secret_basic` token endpoint
auth methods
- Automatic refresh of tokens

### Authorization Server redirect

As part of the flow for the `authorization_code` grant type, the client
needs to point the user to the authorization URL, so that they confirm
the access request. This design proposes that it will be supported via a
pluggable dependency that will initiate this process in an
application-specific way:

```go
type AuthorizationCodeHandler struct {
	...

	// AuthorizationCodeFetcher is a required function called to initiate the authorization flow.
	// It is responsible for opening the URL in a browser for the user to start the authorization process.
	// It should return the authorization code and state once the Authorization Server
	// redirects back to the RedirectURL.
	AuthorizationCodeFetcher func(ctx context.Context, args *AuthorizationArgs) (*AuthorizationResult, error)

	...
}
```

The function is supposed perform any application specific logic to handle the redirect
from the Authorization Server and return the resulting code and state.

## `oauthex` changes

Some additional/adjusted building blocks that may be useful when
creating `OAuthHandler` implementations are present in the `oauthex`
package:

- `auth_meta.go`: Authorization Server Metadata utilities:
  - `GetAuthServerMeta`: get Authorization Server Metadata from a URL
- `dcr.go`: Dynamic Client Registration utilities (unchanged)
- `resource_meta.go`: Protected Resource Metadata utilities:
- `GetProtectedResourceMetadata`: get Protected Resource Metadata from a
URL
  - `ParseWWWAuthenticate` – as per name (unchanged)
- [deprecated] `GetProtectedResourceMetadataFromID` and
`GetProtectedResourceMetadataFromHeader` in favor of more generic
`GetProtectedResourceMetadata`. They will be kept under the Go build tag
and removed one version of the SDK in the future.

mcp/streamable_test.go

@@ -1666,55 +1666,6 @@ func textContent(t *testing.T, res *CallToolResult) string {
 	return text.Text
 }
 
-func TestTokenInfo(t *testing.T) {
-	oldAuth := testAuth.Load()
-	defer testAuth.Store(oldAuth)
-	testAuth.Store(true)
-	ctx := context.Background()
-
-	// Create a server with a tool that returns TokenInfo.
-	tokenInfo := func(ctx context.Context, req *CallToolRequest, _ struct{}) (*CallToolResult, any, error) {
-		return &CallToolResult{Content: []Content{&TextContent{Text: fmt.Sprintf("%v", req.Extra.TokenInfo)}}}, nil, nil
-	}
-	server := NewServer(testImpl, nil)
-	AddTool(server, &Tool{Name: "tokenInfo", Description: "return token info"}, tokenInfo)
-
-	streamHandler := NewStreamableHTTPHandler(func(req *http.Request) *Server { return server }, nil)
-	verifier := func(context.Context, string, *http.Request) (*auth.TokenInfo, error) {
-		return &auth.TokenInfo{
-			Scopes: []string{"scope"},
-			// Expiration is far, far in the future.
-			Expiration: time.Date(5000, 1, 2, 3, 4, 5, 0, time.UTC),
-		}, nil
-	}
-	handler := auth.RequireBearerToken(verifier, nil)(streamHandler)
-	httpServer := httptest.NewServer(mustNotPanic(t, handler))
-	defer httpServer.Close()
-
-	transport := &StreamableClientTransport{Endpoint: httpServer.URL}
-	client := NewClient(testImpl, nil)
-	session, err := client.Connect(ctx, transport, nil)
-	if err != nil {
-		t.Fatalf("client.Connect() failed: %v", err)
-	}
-	defer session.Close()
-
-	res, err := session.CallTool(ctx, &CallToolParams{Name: "tokenInfo"})
-	if err != nil {
-		t.Fatal(err)
-	}
-	if len(res.Content) == 0 {
-		t.Fatal("missing content")
-	}
-	tc, ok := res.Content[0].(*TextContent)
-	if !ok {
-		t.Fatal("not TextContent")
-	}
-	if g, w := tc.Text, "&{[scope] 5000-01-02 03:04:05 +0000 UTC  map[]}"; g != w {
-		t.Errorf("got %q, want %q", g, w)
-	}
-}
-
 func TestSessionHijackingPrevention(t *testing.T) {
 	// This test verifies that sessions bound to a user ID cannot be accessed
 	// by a different user (session hijacking prevention).