refactor: Use Pydantic discriminated union for GraderSpec types

[mattzh72]

• Split GraderSpec into ToolGraderSpec, ModelJudgeGraderSpec, LettaJudgeGraderSpec with shared BaseGraderSpec
• Union type discriminated by kind field using Annotated[Union[...], Field(discriminator='kind')]
• Validation logic moved from init to @model_validator per class
• Required fields now typed correctly (not Optional when required)
• Updated runner.py to use isinstance() checks instead of kind == comparisons
• Updated tests/test_examples_e2e_live.py and visualization/factory.py to use isinstance()
• Backward compatible: YAML configs unchanged