test: add comprehensive unit tests for TriggerChatTransport

Tests cover:
- Constructor with required and optional options
- sendMessages triggering task and returning UIMessageChunk stream
- Correct payload structure sent to trigger API
- Custom streamKey in stream URL
- Extra headers propagation
- reconnectToStream with existing and non-existing sessions
- createChatTransport factory function
- Error handling for API failures
- regenerate-message trigger type

Co-authored-by: Eric Allam <eric@trigger.dev>

Author: cursoragent