Add toJSON() method to OAuthSession interface

Ensures compatibility with hono-oauth-sessions SessionInterface

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[e-mail]>

deno.json

···
 {
   "name": "@tijs/oauth-client-deno",
-  "version": "1.0.2",
+  "version": "1.0.3",
   "description": "AT Protocol OAuth client for Deno - handle-focused alternative to @atproto/oauth-client-node with Web Crypto API compatibility",
   "license": "MIT",
   "repository": {

src/types.ts

···
    * Make authenticated request with automatic DPoP header
    */
   makeRequest(method: string, url: string, options?: RequestInit): Promise<Response>;
+
+  /**
+   * Serialize session data for storage
+   */
+  toJSON(): SessionData;
 }