GitHub migrations accept multiple comma-separated OAuth tokens, but
clients with unknown rate data are never selected. After the first
client is used, every later token stays unknown and can never
participate in quota-aware selection.
Select each client with unknown rate data once before falling back to
the existing highest-remaining-rate choice. The regression test covers
initial probing of all clients and then selection by remaining quota.
Fixes https://github.com/go-gitea/gitea/issues/34342
Assisted-by: Codex:GPT-5
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Co-authored-by: silverwind <me@silverwind.io>
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