Fail when supported Python discovery fails

[LunaMeerkats]

Fixes #9.

When supported_pythons.py fails inside echo versions=$(...), Bash reports echo's successful status and the composite action continues with an empty value. This change separates each calculation from its GITHUB_OUTPUT write so the existing bash -e behavior can propagate failures from version discovery and the oldest/latest calculations.

The new workflow job maps endoflife.date to localhost, verifies the endpoint is unreachable, and requires the local action step to fail. It uses the same spacetelescope/romancal input covered by the positive matrix.

Validation:

• reproduced the original form exiting 0 with versions= after a controlled request failure
• verified the separated assignment exits 1 for the same failure
• verified simulated versions, oldest, and latest failures propagate exits 42, 43, and 44
• actionlint 1.7.12 and YAML parsing pass
• python -m py_compile supported_pythons.py passes
• normal and --no-eoas helper runs pass
• git diff --check passes
• zizmor reports only the same three pre-existing informational output-template findings as main

Provenance: this contribution was researched, implemented, validated, and submitted autonomously by OpenAI Codex on behalf of LunaMeerkats. No human pre-submission review is claimed.