fix(openai): honor passthrough over model_mapping in IsModelSupported (#4936)

An OpenAI account with auto-passthrough enabled (extra.openai_passthrough=true,
"replace auth only, allow all models") was still filtered out during account
selection when it had a non-empty credentials.model_mapping that did not list the
requested model. isOpenAICompatibleAccountEligibleForRequest calls
Account.IsModelSupported directly, and IsModelSupported only bypassed the mapping
whitelist for passthrough accounts when the mapping was empty. A leftover mapping
(common after switching an account from whitelist mode to passthrough) therefore
excluded the account -> zero candidates -> ErrNoAvailableAccounts, and the client
saw 404 "Model ... is not supported by any configured account in this group" even
though a direct account test with the same model succeeded (that path already
honored passthrough via isModelSupportedByAccount).

Fix: short-circuit passthrough at the top of Account.IsModelSupported, before the
model_mapping check, so it is consistent with isModelSupportedByAccount. Add a
regression test covering passthrough + non-empty leftover mapping.

backend/internal/service/account.go

@@ -779,9 +779,16 @@ func resolveRequestedModelInMapping(mapping map[string]string, requestedModel st
 // 请求卡死在该账号上、无法 failover 到真正支持该模型的 API Key 账号（#3662）。
 // 未知/自定义别名仍保持允许（兼容渠道级映射），见 isOpenAIOAuthServableModel。
 func (a *Account) IsModelSupported(requestedModel string) bool {
+	// 透传模式仅替换认证、模型语义完全交由上游决定，因此放行所有模型。
+	// 该短路必须在 model_mapping 判定之前：账号从"白名单模式"切换到透传后，
+	// credentials 里常残留旧的非空 model_mapping，若不在此放行，透传账号会被
+	// model_mapping 白名单错误排除出候选集，导致 no available accounts / 404（issue #4936）。
+	if a.IsOpenAIPassthroughEnabled() {
+		return true
+	}
 	mapping := a.GetModelMapping()
 	if len(mapping) == 0 {
-		if a.IsOpenAIOAuth() && !a.IsOpenAIPassthroughEnabled() {
+		if a.IsOpenAIOAuth() {
 			return isOpenAIOAuthServableModel(requestedModel)
 		}
 		return true // 无映射 = 允许所有

backend/internal/service/openai_oauth_model_support_test.go

@@ -81,6 +81,20 @@ func TestIsModelSupported_OpenAIOAuthPassthroughAllowsAll(t *testing.T) {
 	require.True(t, account.IsModelSupported("deepseek-v4"))
 }
 
+func TestIsModelSupported_OpenAIOAuthPassthroughIgnoresLeftoverMapping(t *testing.T) {
+	account := newOpenAIOAuthAccountForModelTest()
+	account.Extra = map[string]any{"openai_passthrough": true}
+	// 账号从"白名单模式"切到透传后，credentials 里常残留旧的非空 model_mapping。
+	// 透传应无视该白名单，放行不在其中的模型（issue #4936）；否则透传账号会被
+	// 调度期的 IsModelSupported 排除，客户端收到 404 "not supported by any account"。
+	account.Credentials = map[string]any{
+		"model_mapping": map[string]any{"gpt-5.4": "gpt-5.4"},
+	}
+
+	require.True(t, account.IsModelSupported("gpt-5.6-sol"), "透传应放行不在残留白名单中的新模型")
+	require.True(t, account.IsModelSupported("deepseek-v4"), "透传应放行任意模型")
+}
+
 func TestIsModelSupported_OpenAIAPIKeyEmptyMappingAllowsAll(t *testing.T) {
 	account := &Account{
 		ID:       2,