fix: restore OAuth 401 temp-unschedulable for Gemini, update Antigravity tests

The 403 detection PR changed the 401 handler condition from
`account.Type == AccountTypeOAuth` to
`account.Type == AccountTypeOAuth && account.Platform == PlatformOpenAI`,
which accidentally excluded Gemini OAuth from the temp-unschedulable path.

Fix: use `!= PlatformAntigravity` instead, preserving Gemini behavior
while correctly excluding Antigravity (whose 401 is handled by
applyErrorPolicy's temp_unschedulable_rules).

Update tests to reflect Antigravity's new 401 semantics:
- HandleUpstreamError: Antigravity OAuth 401 now uses SetError
- CheckErrorPolicy: Antigravity 401 second hit stays TempUnscheduled
- DB fallback: split into Gemini (escalates) and Antigravity (stays temp)

backend/internal/service/ratelimit_service.go

@@ -149,9 +149,9 @@ func (s *RateLimitService) HandleUpstreamError(ctx context.Context, account *Acc
 		}
 		// 其他 400 错误（如参数问题）不处理，不禁用账号
 	case 401:
-		// OpenAI OAuth 账号在 401 错误时临时不可调度；其他平台 OAuth 账号保持原有 SetError 行为
-		// （Antigravity 主流程不走此路径，其 401 由 applyErrorPolicy 的 temp_unschedulable_rules 自行控制）
-		if account.Type == AccountTypeOAuth && account.Platform == PlatformOpenAI {
+		// OAuth 账号在 401 错误时临时不可调度（给 token 刷新窗口）；非 OAuth 账号保持原有 SetError 行为。
+		// Antigravity 除外：其 401 由 applyErrorPolicy 的 temp_unschedulable_rules 自行控制。
+		if account.Type == AccountTypeOAuth && account.Platform != PlatformAntigravity {
 			// 1. 失效缓存
 			if s.tokenCacheInvalidator != nil {
 				if err := s.tokenCacheInvalidator.InvalidateToken(ctx, account); err != nil {
@@ -183,7 +183,7 @@ func (s *RateLimitService) HandleUpstreamError(ctx context.Context, account *Acc
 			}
 			shouldDisable = true
 		} else {
-			// 非 OAuth 账号（APIKey）：保持原有 SetError 行为
+			// 非 OAuth / Antigravity OAuth：保持 SetError 行为
 			msg := "Authentication failed (401): invalid or expired credentials"
 			if upstreamMsg != "" {
 				msg = "Authentication failed (401): " + upstreamMsg