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/error_policy_test.go

@@ -110,7 +110,9 @@ func TestCheckErrorPolicy(t *testing.T) {
 			expected:   ErrorPolicyTempUnscheduled,
 		},
 		{
-			name: "temp_unschedulable_401_second_hit_upgrades_to_none",
+			// Antigravity 401 不走升级逻辑（由 applyErrorPolicy 的 temp_unschedulable_rules 自行控制），
+			// second hit 仍然返回 TempUnscheduled。
+			name: "temp_unschedulable_401_second_hit_antigravity_stays_temp",
 			account: &Account{
 				ID:                      15,
 				Type:                    AccountTypeOAuth,
@@ -129,7 +131,7 @@ func TestCheckErrorPolicy(t *testing.T) {
 			},
 			statusCode: 401,
 			body:       []byte(`unauthorized`),
-			expected:   ErrorPolicyNone,
+			expected:   ErrorPolicyTempUnscheduled,
 		},
 		{
 			name: "temp_unschedulable_body_miss_returns_none",