Add manual trigger file generation in codegen

[devin-ai-integration]

Summary

Adds support for generating Python trigger files for MANUAL triggers in codegen, enabling workflows to use manual triggers without requiring ENTRYPOINT nodes. This follows the same pattern as SCHEDULED, INTEGRATION, and CHAT_MESSAGE triggers.

Key changes:

• New ManualTrigger generator class that extends BaseTrigger
• Updated generateTriggerFiles() to handle MANUAL trigger type
• Updated getTriggerClassInfo() to return generated trigger path instead of built-in path
• Fixed ManualTriggerSerializer to not include type field in objectSchema (matches pattern of other trigger serializers where unionSchema discriminator handles type separately)

Review & Testing Checklist for Human

• Verify the generated trigger file format matches expected pattern (see snapshot in generate-code.test.ts.snap)
• Confirm the serializer change doesn't break deserialization - the type field removal follows the same pattern as other trigger serializers
• Test round-trip: create a workflow with ManualTrigger in Python, serialize to JSON, verify it matches expected format

Test Plan

1. Run codegen tests: cd ee/codegen && npm test
2. Run Python integration tests: poetry run pytest ee/codegen_integration/test_code_to_display.py -k "simple_manual_trigger_workflow"
3. Optionally test end-to-end by creating a workflow with a manual trigger and running codegen on it

Notes

• The generated trigger class name is derived from the display label (e.g., "Manual Trigger" -> ManualTrigger)
• This is the first step toward transitioning workflow sandboxes to use manual triggers instead of entrypoint nodes

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• Requested by: @NgoHarrison
• Session: https://app.devin.ai/sessions/a3b235cfabb04d1dba7f3cd10b60de57

[devin-ai-integration]

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