fix(eval): handle failed inference results without invocations

[he-yufeng]

What changed

• Return a failed EvalCaseResult when inference failed before producing any invocations.
• Preserve the existing session lookup path when a failed inference still has a session id.
• Add a regression test for InferenceResult(status=FAILURE, inferences=None).
• Clean up two existing lint issues in the touched eval test file so the local changed-file ruff check passes.

This prevents the eval runner from replacing the original inference error with TypeError: object of type 'NoneType' has no len().

Fixes #5876

To verify

• .\.venv\Scripts\python.exe -m py_compile src\google\adk\evaluation\local_eval_service.py tests\unittests\evaluation\test_local_eval_service.py
• .\.venv\Scripts\python.exe -m pytest tests\unittests\evaluation\test_local_eval_service.py -k "failed_without_inferences or evaluate_single_inference_result" -q --basetemp .tmp\pytest
• .\.venv\Scripts\python.exe -m ruff check src\google\adk\evaluation\local_eval_service.py tests\unittests\evaluation\test_local_eval_service.py
• .\.venv\Scripts\python.exe -m pyink --check src\google\adk\evaluation\local_eval_service.py tests\unittests\evaluation\test_local_eval_service.py
• git diff --check

[rohityan]

Hi @he-yufeng , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Please fix formatting errors by running autoformat.sh

[he-yufeng]

Thanks for the review. I rebased onto the latest main, ran pyink on the touched files, and also cleaned two existing lint issues in the touched test file so the changed-file checks are clean. Local verification now passes for py_compile, the targeted eval tests, ruff, pyink --check, and git diff --check.

[he-yufeng]

Rebased onto current upstream/main and reran the focused eval validation. Validation: test_local_eval_service.py passed (19 passed); ruff check passed; pyink --check passed; py_compile passed; git diff --check passed. I set PYTHONPATH=src so pytest imports this worktree explicitly.

[rohityan]

Hi @ankursharmas , can you please review this.