fix(examples,docs): from_standard_webhooks_secret requires key_id kwarg

The hello_seller inline comment and the handler-authoring webhooks
table both showed ``from_standard_webhooks_secret(secret)`` without the
``key_id=`` kwarg, which is required by the actual constructor
signature. Adopters copy-pasting either snippet would hit a
``TypeError: missing 1 required keyword-only argument: 'key_id'``.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bokelley

docs/handler-authoring.md

@@ -1055,7 +1055,7 @@ Pick one per `WebhookSender` instance. All three share the same
 |---|---|---|
 | `WebhookSender.from_jwk(jwk)` | RFC 9421 HTTP-signature | AdCP-conformant buyers; spec baseline (`kid`/`alg`/`adcp_use` live in the JWK dict) |
 | `WebhookSender.from_bearer_token(token)` | `Authorization: Bearer` | Simplest; no key management; requires TLS |
-| `WebhookSender.from_standard_webhooks_secret(secret)` | Standard Webhooks v1 | Svix / Resend / standardwebhooks.com receivers |
+| `WebhookSender.from_standard_webhooks_secret(secret, key_id=...)` | Standard Webhooks v1 | Svix / Resend / standardwebhooks.com receivers |
 
 ### Sender vs. supervisor
 

examples/hello_seller.py

@@ -352,7 +352,9 @@ def _get_packages(req: Any) -> list[dict[str, Any]]:
     #   sender = WebhookSender.from_bearer_token(os.environ["WEBHOOK_BEARER_TOKEN"])
     #
     #   # Standard Webhooks v1 — Svix / Resend / standardwebhooks.com interop:
-    #   sender = WebhookSender.from_standard_webhooks_secret(os.environ["WHSEC"])
+    #   sender = WebhookSender.from_standard_webhooks_secret(
+    #       os.environ["WHSEC"], key_id="whsec_v1",
+    #   )
     #
     #   supervisor = InMemoryWebhookDeliverySupervisor(sender=sender)
     #   serve(HelloSeller(), name="hello-seller", webhook_supervisor=supervisor)