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

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

Fixes #558

oauthex/oauth2.go

@@ -0,0 +1,88 @@
+// 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.
+
+// Package oauthex implements extensions to OAuth2.
+
+//go:build mcp_go_client_oauth
+
+package oauthex
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strings"
+)
+
+// prependToPath prepends pre to the path of urlStr.
+// When pre is the well-known path, this is the algorithm specified in both RFC 9728
+// section 3.1 and RFC 8414 section 3.1.
+func prependToPath(urlStr, pre string) (string, error) {
+	u, err := url.Parse(urlStr)
+	if err != nil {
+		return "", err
+	}
+	p := "/" + strings.Trim(pre, "/")
+	if u.Path != "" {
+		p += "/"
+	}
+
+	u.Path = p + strings.TrimLeft(u.Path, "/")
+	return u.String(), nil
+}
+
+// getJSON retrieves JSON and unmarshals JSON from the URL, as specified in both
+// RFC 9728 and RFC 8414.
+// It will not read more than limit bytes from the body.
+func getJSON[T any](ctx context.Context, c *http.Client, url string, limit int64) (*T, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, err
+	}
+	if c == nil {
+		c = http.DefaultClient
+	}
+	res, err := c.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer res.Body.Close()
+
+	// Specs require a 200.
+	if res.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("bad status %s", res.Status)
+	}
+	// Specs require application/json.
+	if ct := res.Header.Get("Content-Type"); ct != "application/json" {
+		return nil, fmt.Errorf("bad content type %q", ct)
+	}
+
+	var t T
+	dec := json.NewDecoder(io.LimitReader(res.Body, limit))
+	if err := dec.Decode(&t); err != nil {
+		return nil, err
+	}
+	return &t, nil
+}
+
+// checkURLScheme ensures that its argument is a valid URL with a scheme
+// that prevents XSS attacks.
+// See #526.
+func checkURLScheme(u string) error {
+	if u == "" {
+		return nil
+	}
+	uu, err := url.Parse(u)
+	if err != nil {
+		return err
+	}
+	scheme := strings.ToLower(uu.Scheme)
+	if scheme == "javascript" || scheme == "data" || scheme == "vbscript" {
+		return fmt.Errorf("URL has disallowed scheme %q", scheme)
+	}
+	return nil
+}