Make log level an enum

[mathislucka]

We should use a StrEnum for the level filter in get_logs here: src/deepset_mcp/api/pipeline/resource.py

The levels are: info, warning, and error.

Make sure to adjust the tools, unit and integration tests accordingly.

View REPO.md for additional testing instructions.

Only make the changes for StrEnum. Do NOT add any other changes.

The only exception is in main.py, the mcp tool should accept a string that we convert to StrEnum.