Fix /list-workflows endpoint

[botanical]

Context

In some stacks, like mcp-test, when you try to use the /list-workflows endpoint, you get an Internal Server Error. The logs indicate that the function isn't returning a dict

{'loc': ('response',), 'msg': 'value is not a valid dict', 'type': 'type_error.dict'}

Upon investigation, the function was returning an encoded dict and the decoded stderr said:

Error: Failed to load all files. For details, run `airflow dags 
list-import-errors`

Acceptance Criteria

• Endpoint is fixed
• Error messaging is added if there is an error so it's easier to debug/handle

[ividito]

Got blocked by #170 trying to debug this. If you're able to authenticate, I would try running dags list-import-errors using the cli-input endpoint (MWAA requires us to remove the airflow prefix for the CLI)