Add codegen support for ChatMessageTrigger

[devin-ai-integration]

Adds TypeScript codegen support for ChatMessageTrigger with exec_config.output, enabling workflows with chat message triggers to be properly code-generated when pulled from the Vellum API. This is a companion to PR #3433 which added Python serialization support.

Key changes:

• Added CHAT_MESSAGE to WorkflowTriggerType enum and ChatMessageTrigger interface
• Added ChatMessageTriggerSerializer to deserialize CHAT_MESSAGE triggers from the API
• Added ChatMessageTrigger generator that creates a subclass with Config.output when exec_config.output is present
• Added ChatMessageTriggerContext so the workflow graph uses the generated subclass
• Updated getTriggerClassInfo to return the generated subclass path for CHAT_MESSAGE triggers

The generated code creates a trigger subclass like:

class ChatMessage(ChatMessageTrigger):
    class Config(ChatMessageTrigger.Config):
        output = LazyReference(lambda: SomeNode.Outputs.result)

Review & Testing Checklist for Human

• Verify the snapshot test output shows correct generated code for NODE_OUTPUT descriptor type
• Test with a ChatMessageTrigger that has exec_config.output set to a non-NODE_OUTPUT type (e.g., CONSTANT_VALUE, WORKFLOW_INPUT) and verify the generated Config.output is correct
• Test end-to-end: Pull a workflow with a ChatMessageTrigger from Vellum and verify the generated code works at runtime

Notes

• Requested by: @vincent0426
• Session: https://app.devin.ai/sessions/294b6db55b1449458441554a8e214077

[devin-ai-integration]

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