Document opt-in received envelope webhook contract

[shalim786]

Adds the opt-in message.received.envelope webhook contract corresponding to agentmail-api PR #1209. The envelope has its own payload schema, because adding it to the full-content MessageReceivedEventType would incorrectly require message content and a thread object. The global subscription event enum includes the new type, and the webhook catalog exposes the separate payload.

Documents cohort-only permission checks, missing authentication-result semantics, metadata limits, and the console/WebSocket activation limitations without announcing general availability or claiming an SDK release.

Validation: pinned Fern 5.44.8 check passed with zero errors; 25 warnings concern existing route overlaps and an unauthenticated redirect check. MCP tool catalog check and git diff check passed.

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✨ API Changes

# API Changelog n/a vs. n/a


## API Changes

### GET /v0/inboxes/{inbox_id}/webhooks
- :warning: added the new `message.received.envelope` enum value to the `webhooks/items/event_types/items/` response property for the response status `200`


### POST /v0/inboxes/{inbox_id}/webhooks
- :warning: added the new `message.received.envelope` enum value to the `event_types/items/` response property for the response status `200`
-  added the new `message.received.envelope` enum value to the request property `event_types/items/`


### GET /v0/inboxes/{inbox_id}/webhooks/{webhook_id}
- :warning: added the new `message.received.envelope` enum value to the `event_types/items/` response property for the response status `200`


### PATCH /v0/inboxes/{inbox_id}/webhooks/{webhook_id}
- :warning: added the new `message.received.envelope` enum value to the `event_types/items/` response property for the response status `200`
-  added the new `message.received.envelope` enum value to the request property `event_types/items/`


### GET /v0/pods/{pod_id}/webhooks
- :warning: added the new `message.received.envelope` enum value to the `webhooks/items/event_types/items/` response property for the response status `200`


### POST /v0/pods/{pod_id}/webhooks
- :warning: added the new `message.received.envelope` enum value to the `event_types/items/` response property for the response status `200`
-  added the new `message.received.envelope` enum value to the request property `allOf[subschema #1: webhooksCreateInboxWebhookRequest]/event_types/items/`


### GET /v0/pods/{pod_id}/webhooks/{webhook_id}
- :warning: added the new `message.received.envelope` enum value to the `event_types/items/` response property for the response status `200`


### PATCH /v0/pods/{pod_id}/webhooks/{webhook_id}
- :warning: added the new `message.received.envelope` enum value to the `event_types/items/` response property for the response status `200`
-  added the new `message.received.envelope` enum value to the request property `allOf[subschema #1: webhooksUpdateInboxWebhookRequest]/event_types/items/`


### GET /v0/webhooks
- :warning: added the new `message.received.envelope` enum value to the `webhooks/items/event_types/items/` response property for the response status `200`


### POST /v0/webhooks
- :warning: added the new `message.received.envelope` enum value to the `event_types/items/` response property for the response status `200`
-  added the new `message.received.envelope` enum value to the request property `allOf[subschema #1: webhooksCreatePodWebhookRequest]/allOf[subschema #1: webhooksCreateInboxWebhookRequest]/event_types/items/`


### GET /v0/webhooks/{webhook_id}
- :warning: added the new `message.received.envelope` enum value to the `event_types/items/` response property for the response status `200`


### PATCH /v0/webhooks/{webhook_id}
- :warning: added the new `message.received.envelope` enum value to the `event_types/items/` response property for the response status `200`
-  added the new `message.received.envelope` enum value to the request property `allOf[subschema #1: webhooksUpdatePodWebhookRequest]/allOf[subschema #1: webhooksUpdateInboxWebhookRequest]/event_types/items/`

💡 Download api-changelog-diff artifact or tag @fern Writer in #github-prs for changelog.

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