oauthex: move internal/oauthex to oauthex (#559)

This PR relocates oauthex and moves DCR logic into dcr.go.

Fixes #558

oauthex/auth_meta_test.go

@@ -0,0 +1,30 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// 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 oauthex
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestAuthMetaParse(t *testing.T) {
+	// Verify that we parse Google's auth server metadata.
+	data, err := os.ReadFile(filepath.FromSlash("testdata/google-auth-meta.json"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var a AuthServerMeta
+	if err := json.Unmarshal(data, &a); err != nil {
+		t.Fatal(err)
+	}
+	// Spot check.
+	if g, w := a.Issuer, "https://accounts.google.com"; g != w {
+		t.Errorf("got %q, want %q", g, w)
+	}
+}