Add broadcast trigger support to APIClient

[hownowstephen]

Summary

• Add TriggerBroadcastRequest class and APIClient#trigger_broadcast for POST /v1/campaigns/{broadcast_id}/triggers
• Supports all audience options: recipients, emails, ids, per_user_data, data_file_url (mutually exclusive)
• Validates broadcast_id is present and integer, rejects multiple audience types
• Follows existing request class pattern (SendPushRequest, etc.)

Replaces #100. Community-requested feature with multiple forks in use.

Test plan

• CI passes on Ruby 3.3, 3.4, 4.0, head
• Correct payload sent to /v1/campaigns/{id}/triggers
• Each audience type works independently
• Validation rejects missing/non-integer broadcast_id
• Validation rejects multiple audience fields
• 400 and 5xx errors handled correctly

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Note

Low Risk
Low risk additive change: introduces a new request type and endpoint wrapper with input validation and thorough tests, without altering existing send flows.

Overview
Adds a new APIClient#trigger_broadcast method and TriggerBroadcastRequest to trigger API-triggered broadcasts via POST /v1/campaigns/{broadcast_id}/triggers.

TriggerBroadcastRequest validates broadcast_id and enforces that only one audience selector (recipients, emails, ids, per_user_data, data_file_url) is provided, while allowing optional data/flag fields. Documentation and specs are updated to cover usage, payload shapes, and error handling (400/5xx).

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