Fix config file allowing partial overrides for judge/synthetic_user

[richardkiene]

Problem

The judge: and synthetic_user: sections in mcprobe.yaml required provider and model fields, even when the user just wanted to add extra_instructions and inherit everything else from the shared llm: config.

Error:

judge.provider
  Field required [type=missing, input_value={'extra_instructions': '...'}, input_type=dict]

Solution

Added FileLLMConfigOverride model with all optional fields, similar to ScenarioLLMOverride. This allows partial overrides in the config file.

Example

Now this works:

llm:
  provider: ollama
  model: llama3.2

judge:
  extra_instructions: |
    Custom judge instructions that inherit provider/model from llm section

Test plan

• All 258 unit tests pass
• Ruff linting passes
• Mypy type checking passes