Revert Summary Metrics and Expand Test Coverage to Stabilize Nightly/Main CI

[markurtz]

Summary:

Reverts the summary metrics logic in src/guidellm/core/report.py and src/guidellm/core/result.py that was landed due to failing tests. Additionally, test cases are expanded to ensure full coverage of these changes and to stabilize the nightly and main CI pipelines.

Details:

• Replaced direct token statistics (prompt_token, output_token) with distribution-based calculations (prompt_token_distribution, output_token_distribution).
• Modified percentile handling for request latency, time-to-first-token (TTFT), and inter-token latency (ITL) to improve performance summary accuracy.
• Removed computed_field annotations for several properties in src/guidellm/core/result.py.
• Updated tests in tests/unit/core/test_report.py from @pytest.mark.regression to @pytest.mark.sanity to better align with the testing standards.

Test Plan:

• Unit tests have been added/updated to verify:
  • Correctness of the refactored token statistics and distribution calculations.
  • Accurate summary report generation for benchmarks.
  • Full compatibility with existing functionality.
• Verified passing CI/CD pipeline, ensuring no regressions.

[parfeniukink]

Looks good