Fix DUO_WORKFLOW_DEFINITION env var for custom flows

What does this MR do and why?

StartWorkflowService#resolved_workflow_definition builds the DUO_WORKFLOW_DEFINITION env var by concatenating flow_config_id and flow_config_schema_version. For foundational flows this works fine because FoundationalFlowStartParamsResolver always sets flow_config_id.

Custom (non-foundational) flows from AI Catalog don't go through that resolver, so flow_config_id is blank. That means DUO_WORKFLOW_DEFINITION ends up as "/v1" instead of something meaningful.

This MR adds a fallback: when flow_config_id is blank, we use the stable workflow_definition stored on the workflow record (e.g. ai_catalog_agent). Foundational flows are not affected since they always have flow_config_id set.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

Verification steps

Setup

1. Make sure you have a custom flow (non-foundational) set up in AI Catalog
2. Trigger the custom flow from an issue comment via @mention (e.g. @ai-my-helpful-flow hi)
3. Once the workflow starts, open the CI job page for the workflow run

Check the env var

4. In the job log, find the echo $DUO_WORKFLOW_DEFINITION line
5. Confirm it shows ai_catalog_agent (not /v1)
6. The workflow should run and complete successfully

Run specs

bin/rspec ee/spec/services/ai/duo_workflows/start_workflow_service_spec.rb

MR acceptance checklist

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