Harden zero-test validation evidence

[yyjeqhc]

Summary

• Treat reliably identified generic cargo test executions that ran zero tests as inconclusive validation proof without changing process success.
• Preserve explicit Cargo validation execution policy (require_tests / no_run) durably through Runner Job lifecycle and reconciliation behind the additive structured_cargo_test_execution_policy capability.
• Clarify the public cargo_test contract for omitted/false/true require_tests and no_run=true.

Review

Fresh independent review checked the three prior findings, historical failure reconciliation, rolling-upgrade fail-closed behavior, durable Some(false) round-trip, capability inference/sticky downgrade classification, and model-facing schema wording. No remaining correctness blocker found.

Rebased cleanly onto latest main (70527035, #312); #312 only changes Plugin docs/example files and does not overlap this range.

Validation

• cargo test -p webcodex-validation — 90 passed
• cargo test -p webcodex-runner-registry — 240 passed
• cargo test -p webcodex-tool-contracts — 88 passed
• implementation handoff also reports affected package checks, cargo fmt --all -- --check, and git diff --check passing

No deploy/restart/release changes.