feat(benchmark): Add -N, --attempts option for multiple attempts per challenge

LLMs are probabilistic systems. Reproducibility of completions is not guaranteed. It only makes sense to account for this, by running challenges multiple times to obtain a success ratio rather than a boolean success/failure result.

Changes:
- Add `-N`, `--attempts` option to CLI and `attempts_per_challenge` parameter to `main.py:run_benchmark`.
- Add dynamic `i_attempt` fixture through `pytest_generate_tests` hook in conftest.py to achieve multiple runs per challenge.
- Modify `pytest_runtest_makereport` hook in conftest.py to handle multiple reporting calls per challenge.
- Refactor report_types.py, reports.py, process_report.ty to allow multiple results per challenge.
   - Calculate `success_percentage` from results of the current run, rather than all known results ever.
   - Add docstrings to a number of models in report_types.py.
   - Allow `None` as a success value, e.g. for runs that did not render any results before being cut off.
- Make SingletonReportManager thread-safe.

benchmark/agbenchmark/challenges/builtin.py

@@ -155,7 +155,10 @@ class BuiltinChallenge(BaseChallenge):
 
     @pytest.mark.asyncio
     async def test_method(
-        self, config: AgentBenchmarkConfig, request: pytest.FixtureRequest
+        self,
+        config: AgentBenchmarkConfig,
+        request: pytest.FixtureRequest,
+        i_attempt: int,
     ) -> None:
         if os.environ.get("HELICONE_API_KEY"):
             from helicone.lock import HeliconeLockManager