fix(webhook): thread scheduled_at into the review-hold event payloads

[AmirF194]

Summary

buildPendingApprovalEvent and buildApprovedEvent (internal/agent/webhooks_api.go) never read msg.ScheduledAt / sent.ScheduledAt, so a webhook-only subscriber of email.review_requested / email.review_approved cannot tell a deferred send from an immediate one without a follow-up GET on the message. Adds scheduled_at to both payloads when the row carries a schedule, the same optional-field pattern the same builders already use for provider_message_id and lifecycle_transitions.

On the issue's third fix step (regenerate the OpenAPI + generated SDK event models): email.review_requested and email.review_approved are deliberately kept as map[string]any at their trigger sites and are not registered as OpenAPI component schemas (internal/eventpayload/payloads.go's package doc: these two stay untyped "until their shape settles"). There is no generated model or golden fixture for either event, so there is nothing to regenerate; make generate-sdk-check and the OpenAPI contract gates don't cover this payload.

Client surface checklist

Not applicable: this is an additive field on an already-untyped, beta webhook payload, not a typed API or client-SDK surface (see note above).

Test plan

• TestBuildPendingApprovalEvent_CarriesScheduledAt / TestBuildApprovedEvent_CarriesScheduledAt: fail on main, pass on this branch.
• TestBuildPendingApprovalEvent_OmitsScheduledAtForImmediateSend / TestBuildApprovedEvent_OmitsScheduledAtForImmediateSend: cover the unset case.
• go test ./... (full suite, go1.26 per CI) and make fmt-check both pass.
• Did not check: the coverage-gate job (make cover) needs Postgres and doesn't gate internal/agent, so I didn't run it.

Fixes #836

[jiashuoz]

Reviewed the changes and verified that the required CI checks pass.

[AmirF194]

Thanks for the quick review and merge.