Fix inspect-ai GenerateConfig field names in lighteval eval

[harshasiddartha]

Fixes #1327.

lighteval eval crashes at startup on a fresh install, for every model and task:

ValidationError: 1 validation error for GenerateConfig
  Value error, Unknown GenerateConfig field(s): frequence_penalty, log_probs, response_format.

Three of the generation parameters main_inspect.py forwards to inspect_ai.eval_set are not GenerateConfig fields:

forwarded	actual field
frequence_penalty	frequency_penalty
log_probs	logprobs
response_format (str)	response_schema (ResponseSchema)

Worth noting these names were never correct — they are wrong on inspect-ai==0.3.140 too, which is the declared minimum. Older releases just dropped unknown fields silently, so --frequence-penalty, --log-probs and --response-format were accepted on the command line and then had no effect. Current releases validate, which turns that silent no-op into a hard startup failure. So this is a rename rather than a compatibility shim, and it works across the whole supported inspect-ai range — no upper bound needed.

Changes

• Rename the three parameters in the eval command and in InspectAIModelConfig, which mirrors them.
• --response-schema now parses its JSON string into a ResponseSchema, accepting either a bare JSON schema (wrapped under the default name response) or a full ResponseSchema object.
• Add tests/unit/test_main_inspect.py, which asserts every generation parameter is a real GenerateConfig field and is exposed on the CLI. That's the regression guard — a name drifting out of sync now fails in CI instead of at a user's startup. Plus coverage for the schema parsing.

Verification

The command from the issue now runs to completion on current inspect-ai (0.3.258):

lighteval eval mockllm/model "lighteval|gsm8k|0" --max-samples 2 --log-dir out
...
Completed all tasks in 'out' successfully

|    Model    |gsm8k|
|-------------|----:|
|mockllm/model|    0|

tests/unit/test_main_inspect.py passes (8 tests); ruff check and ruff format --check are clean on the touched files.

Note on the CLI rename

--frequence-penalty → --frequency-penalty and --log-probs → --logprobs are user-visible flag renames. Since neither flag has ever had any effect (silently ignored before, crashing now), there is nothing depending on the old spellings. Happy to add hidden aliases for the old names if you'd rather not break them outright.