Sends offer to carriers using the default template for the channel.

**Auto-Creation:**
- If the offer doesn't exist, it will be automatically created using calculated defaults
- Requires `integration_connection_id` in request body when offer doesn't exist
- Pass the literal string `new` as `offer_id` to skip the existing-offer lookup and force
  creation from the request payload (used by the compose flow before any offer is persisted)
- When `min_load_offer_carrier_rate_amount` is provided in the body it overrides any saved value;
  the LoadOffer is created or updated at send time using that amount

**Template System:**
- If `template_id` is provided, uses that specific template
- If `subject` and `body` are provided, uses them directly (no template)
- Otherwise, uses the default template for the channel
- Template variables are automatically filled with load and carrier data from Vortex

**Template Variables:**
Templates support {{variable}} syntax filled from LoadSchema and CarrierSchema:
- {{load.external_reference_id}}, {{load.weight}}, {{load.equipment_type}}
- {{carrier.name}}, {{carrier.dot_number}}
- {{load.customer.name}}, {{load.stops.0.city}}, etc.

**Channel Types:**
- EMAIL: Sends via workspace email integration
- SMS: Sends via SMS integration (future)
- WHATSAPP: Sends via WhatsApp integration (future)