Validator fail-open re-run failures print a full traceback

[hertznsk]

Description

When a semantic validator rejects an agent output and the validator-triggered primary re-run fails, Conductor correctly fails open and keeps the original output. However, the expected fail-open path is logged with exc_info=True, so users see a full Python traceback even though the workflow continues successfully.

This makes a handled validator recovery failure look like an unhandled workflow crash and adds noisy implementation details to normal CLI output.

Reproduction

On current main (f39bd63c3f8fe540f164102ebedb598526b2f686):

1. Configure an agent with validator.max_retries: 1.
2. Return passed: false from the validator.
3. Make the validator-triggered primary re-run raise a provider/runtime exception.
4. Run the workflow with warning logs visible.

The existing regression scenario demonstrates the behavior directly:

uv run pytest tests/test_engine/test_validator_integration.py::TestValidatorCostAndFailurePaths::test_rerun_failure_keeps_original_no_double_count_and_emits_event -q -s --log-cli-level=WARNING

Observed output includes:

WARNING conductor.engine.workflow: Validator re-run failed for 'reviewer'; using original output
Traceback (most recent call last):
  ...
RuntimeError: rerun boom
PASSED

Expected behavior

The workflow should still fail open and retain the original output, but the handled re-run failure should be reported as a concise warning without a full traceback in normal user-facing logs. Diagnostic exception details can remain available at debug level or in structured diagnostic metadata/logs.

Actual behavior

WorkflowEngine._apply_validator() catches the exception and returns the original output, but logs the warning with exc_info=True in src/conductor/engine/workflow.py. This prints the entire traceback at WARNING level despite the failure being handled.

Additional context

The existing test verifies output preservation, usage accounting, and the rerun_errored event, but does not assert that the user-facing fail-open path avoids traceback noise.