Fix deterministic convergence report test clock

[cbusillo]

Why

The compact-artifact test uses fixed August 22 timestamps but allowed the report entry point to read the wall clock. Once those fixtures crossed the production freshness thresholds, the test began emitting alerts and blocking unrelated pull requests.

Related to #634.

What changed

• Wrap the report module's datetime dependency in the existing test.
• Return the existing fixed NOW from datetime.now() while preserving real datetime.fromisoformat() behavior.
• Leave production report behavior, thresholds, workflow scheduling, and fixtures unchanged.

Validation

• python3 -m unittest test_upstream_convergence_report.TestUpstreamConvergenceReport.test_main_writes_compact_artifact
• python3 -m unittest discover -s .github/scripts -p 'test_*.py' (309 tests)