Enterprise managed authorization (#770)

`auth`, `oauthex`: implement Enterprise Managed Authorization (SEP-990)
This PR implements Enterprise Managed Authorization (SEP-990) for the Go
MCP SDK, enabling MCP Clients and Servers to leverage enterprise
Identity Providers for seamless authorization without requiring users to
authenticate separately to each MCP Server.
 Overview
Enterprise Managed Authorization follows the Identity Assertion
Authorization Grant specification
(draft-ietf-oauth-identity-assertion-authz-grant), implementing a
three-step flow:
1. **Single Sign-On (SSO)**: User authenticates to the MCP Client via
enterprise IdP (Okta, Auth0, Azure AD, etc.)
2. **Token Exchange (RFC 8693)**: Client exchanges ID Token for Identity
Assertion JWT Authorization Grant (ID-JAG) at the IdP
3. **JWT Bearer Grant (RFC 7523)**: Client exchanges ID-JAG for Access
Token at the MCP Server
This enables:
- **For end users**: Single sign-on across MCP Clients and Servers—no
manual connection/authorization per server
- **For enterprise admins**: Centralized visibility and control over
which MCP Servers can be used within the organization
- **For MCP clients**: Automatic token acquisition without user
interaction for each server

Closes:
[#628](https://github.com/modelcontextprotocol/go-sdk/issues/628)

---------

Co-authored-by: Maciej Kisiel <mkisiel@google.com>

auth/client.go

@@ -22,8 +22,6 @@ import (
 // [github.com/modelcontextprotocol/go-sdk/mcp.StreamableClientTransport]
 // for an example.
 type OAuthHandler interface {
-	isOAuthHandler()
-
 	// 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.