extproc: pick random backend when multiple unweighted ones (#402)

**Commit Message**

Right now if there are multiple, unweighted `backendRefs` in a
[AIGatewayRouteRule], the first one is always returned. The correct
behavior is to return a random one since they're essentially all
weighted the same.

[AIGatewayRouteRule]:
https://envoy-ai-gateway.netlify.app/docs/api/#aigatewayrouterule

**Related Issues/PRs (if applicable)**

contributes to #53

Signed-off-by: David Xia <[email protected]>

Author: davidxia

internal/extproc/router/router.go

@@ -57,9 +57,12 @@ func (r *router) selectBackendFromRule(rule *filterapi.RouteRule) (backend *filt
 	for _, b := range rule.Backends {
 		totalWeight += b.Weight
 	}
+
+	// Pick a random backend if none of them have a weight.
 	if totalWeight == 0 {
-		return &rule.Backends[0]
+		return &rule.Backends[r.rng.Intn(len(rule.Backends))]
 	}
+
 	selected := r.rng.Intn(totalWeight)
 	for i := range rule.Backends {
 		b := &rule.Backends[i]

internal/extproc/router/router_test.go

@@ -51,6 +51,15 @@ func TestRouter_Calculate(t *testing.T) {
 					{Name: "x-model-name", Value: "llama3.3333"},
 				},
 			},
+			{
+				Backends: []filterapi.Backend{
+					{Name: "baz", Schema: outSchema},
+					{Name: "qux", Schema: outSchema},
+				},
+				Headers: []filterapi.HeaderMatch{
+					{Name: "x-model-name", Value: "o1"},
+				},
+			},
 			{
 				Backends: []filterapi.Backend{
 					{Name: "openai", Schema: outSchema},
@@ -76,6 +85,18 @@ func TestRouter_Calculate(t *testing.T) {
 		require.Equal(t, "openai", b.Name)
 		require.Equal(t, outSchema, b.Schema)
 	})
+	t.Run("matching rule - multiple unweighted backend choices", func(t *testing.T) {
+		chosenNames := make(map[string]int)
+		for i := 0; i < 1000; i++ {
+			b, err := r.Calculate(map[string]string{"x-model-name": "o1"})
+			require.NoError(t, err)
+			chosenNames[b.Name]++
+			require.Contains(t, []string{"baz", "qux"}, b.Name)
+			require.Equal(t, outSchema, b.Schema)
+		}
+		require.InDelta(t, 500, chosenNames["qux"], 50)
+		require.InDelta(t, 500, chosenNames["baz"], 50)
+	})
 	t.Run("matching rule - multiple backend choices", func(t *testing.T) {
 		chosenNames := make(map[string]int)
 		for i := 0; i < 1000; i++ {