nit(examples): docstring run command + explicit UserWarning category

Pre-PR review nits:
- Run:: example shows <your-token> placeholder instead of the dev default
- warnings.warn gains category=UserWarning to match conventional style

https://claude.ai/code/session_01Ay2SJvRpBg8EP9nG9DgQMy

Author: claude

examples/hello_seller_with_webhooks.py

@@ -7,7 +7,7 @@
 
 Run::
 
-    WEBHOOK_BEARER_TOKEN=dev-fixture-token uv run python examples/hello_seller_with_webhooks.py
+    WEBHOOK_BEARER_TOKEN=<your-token> uv run python examples/hello_seller_with_webhooks.py
 
 The server boots on http://localhost:3001/mcp.  Any buyer that registers
 ``push_notification_config.url`` on a ``create_media_buy`` request receives a
@@ -42,6 +42,7 @@
         warnings.warn(
             "WEBHOOK_BEARER_TOKEN is not set; using 'dev-fixture-token'. "
             "Set WEBHOOK_BEARER_TOKEN=<real-token> before connecting real buyers.",
+            category=UserWarning,
             stacklevel=1,
         )
         token = "dev-fixture-token"