edr fails to start without PyYAML, which it imports but doesn't declare #2398

Describe the bug
edr fails on startup when PyYAML isn't installed. elementary/clients/dbt/command_line_dbt_runner.py imports yaml, but elementary-data doesn't declare PyYAML. It used to come in with dbt-core, so it went missing once 0.26.1 made dbt-core optional (#2354). dbt v2 doesn't bring it either, since dbt and dbt-oss 2.0.8 depend only on mashumaro[msgpack].

  File ".../site-packages/elementary/clients/dbt/command_line_dbt_runner.py", line 7, in <module>
    import yaml
ModuleNotFoundError: No module named 'yaml'

To Reproduce

1. Install elementary-data 0.26.1 and dbt 2.0.6 in an environment with nothing else that needs PyYAML
2. Run edr --help

Expected behavior
edr starts, with PyYAML installed as one of its dependencies.

Environment (please complete the following information):

• Elementary CLI (edr) version: 0.26.1, and master declares the same dependencies
• Elementary dbt package version: 0.26.1
• dbt version you're using: 2.0.6
• Data warehouse: Snowflake
• Infrastructure details: Python 3.14, installed with uv

Additional context
We've pinned PyYAML in our own project until this is fixed.

Would you be willing to contribute a fix for this issue?
Yes, #2397.