Cancel a campaign and mark it as canceled.

Cancelable source states: scheduled or paused (all channel types), or started (email only). Any other status (e.g. draft, sent, canceled) returns 409.

Supported cases:

• Any scheduled or paused campaign (all channel types)
• Started email campaigns

Note: For started campaigns, cancellation is best-effort — messages already in flight may still be delivered.

Scopes: campaigns.write