The submit flow discarded the real failure reason and always returned the
generic "Failed to submit benchmark results." to the UI. The most common
cause is the leaderboard's one-per-hour rate limit (HTTP 429), which left
users with no idea why their submission failed or that retrying shortly
would also fail.
- benchmark_controller: return a clear, actionable message. Name the rate
limit explicitly on 429; otherwise pass through the underlying detail
(repository error or a service validation message like "already
submitted"), falling back to the generic only when we have nothing.
- benchmark_service: attach the raw upstream `detail` to the thrown error so
the controller can surface it.
The frontend already renders the server `error` string, so no client change
is needed.
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