fix(ci): fail vector-result merge on vacuous or inconsistent artifacts

[yilirong1992]

merge_vector_results: reject vacuous or self-inconsistent result artifacts

Summary

The vector-result merger trusted every readable artifact. That leaves a false-PASS
hole in CI: a results-<adapter>.json that parses as JSON but carries no actual
conformance data merges into an overall status=pass.

Reproducible on main before this change:

$ echo '{}' > vector-results/results-go.json
$ python3 scripts/merge_vector_results.py --adapters go --result-dir vector-results --output merged.json
Vector status: pass        # 0 checks, 0 failures — green

Concrete ways to hit this:

• the runner is killed after the artifact file is created but before checks run
  (truncated-but-valid JSON, or an empty object),
• an upload/download step swaps in a wrong or stale file,
• a future runner regression emits status: "fail" with malformed counts —
  the merger ignored the status field entirely and only summed failed.

Since branch protection keys on the merged status, any of these silently green-lights
a PR that never ran conformance for that adapter.

What this PR does

Each artifact must now carry real evidence of a run:

1. Non-object payloads ([], a bare string, …) are treated like unreadable
   files and recorded as a runner failure for that adapter.
2. Zero-check artifacts (num_checks == 0, including {}) fail with
   "... reported zero conformance checks". The vector runner always emits at
   least one check (it has an explicit no_checks guard), so a zero-check
   artifact can only mean the run did not happen.
3. Status/count inconsistency (status != "pass" but failed == 0) fails
   instead of being ignored — the merger now honors the artifact's own status
   field as a failure signal.
4. An empty --adapters list produces a failing merge instead of a green
   run with zero checks.

Counting and error propagation for healthy artifacts is unchanged, as is the
$GITHUB_OUTPUT interface consumed by conformance.yml and the monitor workflow.

Tests

New test_merge_vector_results.py (picked up by the existing
unittest discover step in ci.yml) covers: healthy merge, {} artifact,
zero-check artifact, status:"fail" with zero counts, non-object payload,
missing artifact file, and the empty adapter list. Full script suite:
16 tests, OK.