Add automated accessibility suite execution to the standard non-security
end-to-end browser shards so regressions are caught during routine CI runs.
This change is necessary to enforce accessibility checks consistently across
Chromium, Firefox, and WebKit without creating a separate pipeline path.
Behavior impact:
- Non-security shard jobs now run accessibility tests alongside existing suites
- Security-specific job behavior remains unchanged
- Sharding logic remains unchanged, with only test scope expanded
Operational consideration:
- Monitor shard runtime balance after rollout; if sustained skew appears,
split accessibility coverage into its own sharded workflow stage.
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