Add serialization support for ChatMessageTrigger

[devin-ai-integration]

Summary

Adds serialization support for ChatMessageTrigger workflows, including:

• The CHAT_MESSAGE trigger type in WorkflowTriggerType enum
• Attribute serialization with JSON fallback for complex union types that can't be mapped to primitive Vellum variable types
• exec_config.output serialization for ChatMessageTrigger configs

Updates since last revision

• Rebased on main
• Reverted changes to expressions.py and test_attributes_serialization.py per reviewer request

Review & Testing Checklist for Human

• Verify the JSON fallback in get_attribute_type() is appropriate for all attribute types that may appear in ChatMessageTrigger
• Confirm _serialize_chat_message_trigger_exec_config correctly handles cases where Config.output is None
• Test serialization with a real ChatMessageTrigger workflow to ensure the output format matches backend expectations

Notes

• Requested by: @vincent0426
• Session: https://app.devin.ai/sessions/b9b09704bdca438bbc860f1cfa4b8dbf

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