refactor: extract failover error handling into shared HandleFailoverError

- Extract duplicated failover error handling from gateway_handler.go (Gemini-compat & Claude paths) and gemini_v1beta_handler.go into shared failover_loop.go
- Introduce TempUnscheduler interface for testability (GatewayService implicitly satisfies it)
- Add comprehensive unit tests for HandleFailoverError (32 test cases covering all paths)
- Fix golangci-lint issues: errcheck in test type assertion, staticcheck QF1003 if/else→switch

backend/internal/service/error_passthrough_runtime_test.go

@@ -219,7 +219,9 @@ func TestApplyErrorPassthroughRule_SkipMonitoringSetsContextKey(t *testing.T) {
 	assert.True(t, matched)
 	v, exists := c.Get(OpsSkipPassthroughKey)
 	assert.True(t, exists, "OpsSkipPassthroughKey should be set when skip_monitoring=true")
-	assert.True(t, v.(bool))
+	boolVal, ok := v.(bool)
+	assert.True(t, ok, "value should be a bool")
+	assert.True(t, boolVal)
 }
 
 func TestApplyErrorPassthroughRule_NoSkipMonitoringDoesNotSetContextKey(t *testing.T) {