chore: enable ANN ruff ruleset and add missing docstrings for deepeval integration

[julian-risch]

Related Issues

None

Proposed Changes:

• Add ANN (flake8-annotations) to ruff select in pyproject.toml
• Add ANN401 to ruff ignore (allow Any for SDK boundaries)
• Exclude tests/**/* from ANN checks via per-file-ignores
• Add -> None return type to DeepEvalEvaluator.__init__ and _init_backend
• Add -> str to DeepEvalMetric.__str__
• Add -> dict[str, Any] to MetricResult.to_dict
• Add **kwargs: Any and -> None to InputConverters._validate_input_elements
• Add docstrings to MetricResult.to_dict, MetricDescriptor.new, InputConverters.validate_input_parameters, and the InputConverters/OutputConverters converter methods

How did you test it?

Ran tests locally

Notes for the reviewer

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.