fix(webhooks): apply method-level options

[fallintoplace]

Summary

• apply method-level request options when resolving the webhook secret
• preserve method-level precedence over client configuration
• cover method-level secrets and override behavior with regression tests

Why

The webhook verification methods accept request options, but signature verification only applied the service's stored options. As a result, passing option.WithWebhookSecret directly to VerifySignature or Unwrap was ignored and could return the missing-secret error.

Tests

• go test ./...
• ./scripts/lint

[jbeckwith-oai]

Reviewed the option-precedence change and regression coverage. Method-level webhook options are applied after service/client options through the same slices.Concat ordering used by generated request methods, without mutating the stored options. Targeted webhook tests, Go 1.25 compatibility, race tests, vet, lint/build, formatting, and module-tidy checks are clean.