fix(benchmark): surface a clear reason when leaderboard submission fails (#1138)

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.

admin/app/controllers/benchmark_controller.ts

@@ -184,9 +184,22 @@ export default class BenchmarkController {
       // Pass through the status code from the service if available, otherwise default to 400
       const statusCode = (error as any).statusCode || 400
       logger.error({ err: error }, '[BenchmarkController] Benchmark submit failed')
+
+      // Surface a clear, actionable reason to the UI instead of a generic failure.
+      // The rate limiter (429) is the most common cause, so name it explicitly;
+      // otherwise pass through the underlying detail (repository error or a service
+      // validation message) and fall back to a safe generic only when we have none.
+      let errorMessage: string
+      if (statusCode === 429) {
+        errorMessage = 'You can only submit one benchmark per hour. Please wait a bit and try again.'
+      } else {
+        errorMessage =
+          (error as any).detail || (error as any).message || 'Failed to submit benchmark results.'
+      }
+
       return response.status(statusCode).send({
         success: false,
-        error: 'Failed to submit benchmark results.',
+        error: errorMessage,
       })
     }
   }

admin/app/services/benchmark_service.ts

@@ -299,9 +299,11 @@ export class BenchmarkService {
       const statusCode = error.response?.status
       logger.error(`Failed to submit benchmark to repository: ${detail} (Status: ${statusCode})`)
       
-      // Create an error with the status code attached for proper handling upstream
+      // Create an error with the status code and raw detail attached for proper
+      // handling upstream (the controller surfaces `detail` as the user-facing reason).
       const err: any = new Error(`Failed to submit benchmark: ${detail}`)
       err.statusCode = statusCode
+      err.detail = detail
       throw err
     }
   }