fix(migrations): use all configured GitHub tokens (#38841)

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

---------

Co-authored-by: silverwind <me@silverwind.io>

services/migrations/github_test.go

@@ -14,6 +14,7 @@ import (
 	"gitea.dev/models/unittest"
 	base "gitea.dev/modules/migration"
 
+	"github.com/google/go-github/v89/github"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -473,3 +474,25 @@ func TestGithubMultiToken(t *testing.T) {
 		})
 	}
 }
+
+func TestGithubMultiTokenClientSelection(t *testing.T) {
+	downloader := &GithubDownloaderV3{
+		clients: make([]*github.Client, 3),
+		rates:   make([]*github.Rate, 3),
+	}
+
+	downloader.waitAndPickClient(t.Context())
+	assert.Equal(t, 0, downloader.curClientIdx)
+
+	downloader.rates[0] = &github.Rate{Remaining: 100}
+	downloader.waitAndPickClient(t.Context())
+	assert.Equal(t, 1, downloader.curClientIdx)
+
+	downloader.rates[1] = &github.Rate{Remaining: 200}
+	downloader.waitAndPickClient(t.Context())
+	assert.Equal(t, 2, downloader.curClientIdx)
+
+	downloader.rates[2] = &github.Rate{Remaining: 50}
+	downloader.waitAndPickClient(t.Context())
+	assert.Equal(t, 1, downloader.curClientIdx)
+}