fix: resolve backend CI failures

[yangzc2004-bit]

Summary

• fix OpenAI scheduler snapshot account selection when repository dependencies are absent in tests
• satisfy backend golangci-lint errcheck/staticcheck/formatting findings
• keep the PR scoped to backend CI fixes

Verification

• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.9.0 run --timeout=30m
• go test -tags=unit ./...
• go test -tags=integration ./...

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