[management,proxy] Add agentgateway integration (#7274)

* [management] Add agentgateway provider catalog entry

Allow Agent Network providers to target an operator-supplied agentgateway proxy while stamping trusted NetBird identity headers.

Signed-off-by: Daneyon Hansen <daneyon.hansen@solo.io>

* [proxy] Allow trusted Agent Network identity headers

Permit only the built-in identity injector to replace the two reserved agentgateway attribution headers while keeping them blocked for every other middleware.

Signed-off-by: Daneyon Hansen <daneyon.hansen@solo.io>

* [management,proxy] Add multi-vendor gateway routing

Let one Agent Network route declare multiple parser surfaces while preserving the existing singular vendor wire field.

Signed-off-by: Daneyon Hansen <daneyon.hansen@solo.io>

* [management] Update router test for model policies

Signed-off-by: Daneyon Hansen <daneyon.hansen@solo.io>

* [proxy] Cover reserved header policy

Signed-off-by: Daneyon Hansen <daneyon.hansen@solo.io>

* [management] Add agentgateway model discovery

Use agentgateway's OpenAI-compatible models endpoint and omit wildcard patterns until NetBird can authorize and price them consistently.

Signed-off-by: Daneyon Hansen <daneyon.hansen@solo.io>

---------

Signed-off-by: Daneyon Hansen <daneyon.hansen@solo.io>

proxy/internal/middleware/builtin/llm_router/middleware_test.go

@@ -412,6 +412,50 @@ func TestRouter_VendorKeepsOpenAIOffAnthropic(t *testing.T) {
 	assert.Equal(t, "api.openai.com", out.Mutations.RewriteUpstream.Host, "openai vendor must pin to the openai route despite anthropic being declared first")
 }
 
+func TestRouter_MultiVendorGatewayAcceptsBothSurfaces(t *testing.T) {
+	gateway := ProviderRoute{
+		ID:              "agentgateway",
+		Vendors:         []string{"openai", "anthropic"},
+		Models:          nil,
+		AllowedGroupIDs: []string{defaultTestGroup},
+		UpstreamScheme:  "https",
+		UpstreamHost:    "gateway.example.com",
+	}
+	other := ProviderRoute{
+		ID:              "other-vendor",
+		Vendor:          "mistral",
+		Models:          nil,
+		AllowedGroupIDs: []string{defaultTestGroup},
+		UpstreamScheme:  "https",
+		UpstreamHost:    "mistral.example.com",
+	}
+	mw := New(Config{Providers: []ProviderRoute{other, gateway}})
+
+	for _, tc := range []struct {
+		name   string
+		vendor string
+		model  string
+		path   string
+	}{
+		{name: "OpenAI", vendor: "openai", model: "gpt-4o-mini", path: "/v1/chat/completions"},
+		{name: "Anthropic", vendor: "anthropic", model: "claude-sonnet-4-5", path: "/v1/messages"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			out, err := mw.Invoke(context.Background(), newInputVendorModelURL(tc.vendor, tc.model, tc.path))
+			require.NoError(t, err)
+			require.NotNil(t, out)
+			assert.Equal(t, middleware.DecisionAllow, out.Decision,
+				"supported vendor must route through the multi-surface gateway")
+			require.NotNil(t, out.Mutations)
+			require.NotNil(t, out.Mutations.RewriteUpstream)
+			assert.Equal(t, "gateway.example.com", out.Mutations.RewriteUpstream.Host)
+
+			provider, _ := metaValue(t, out.Metadata, middleware.KeyLLMResolvedProviderID)
+			assert.Equal(t, "agentgateway", provider)
+		})
+	}
+}
+
 // TestRouter_VendorAbsentFallsBackToModelPath confirms vendor filtering is
 // inert when the request carries no detected vendor: routing then relies on
 // model/path as before.
@@ -692,6 +736,23 @@ func TestRouter_FactoryRejectsBadJSON(t *testing.T) {
 	require.Error(t, err, "malformed JSON config must be rejected at chain build time")
 }
 
+func TestRouter_FactoryDecodesLegacyAndMultiVendorFields(t *testing.T) {
+	raw := []byte(`{"providers":[` +
+		`{"id":"legacy","vendor":"openai","models":[],"upstream_scheme":"https","upstream_host":"openai.example.com","auth_header_name":"Authorization","auth_header_value":"Bearer legacy","allowed_group_ids":["group"]},` +
+		`{"id":"multi","vendors":["openai","anthropic"],"models":[],"upstream_scheme":"https","upstream_host":"gateway.example.com","auth_header_name":"Authorization","auth_header_value":"Bearer multi","allowed_group_ids":["group"]}` +
+		`]}`)
+
+	resolved, err := Factory{}.New(raw)
+	require.NoError(t, err)
+	router, ok := resolved.(*Middleware)
+	require.True(t, ok, "factory must return the concrete router middleware")
+	require.Len(t, router.cfg.Providers, 2)
+	assert.Equal(t, "openai", router.cfg.Providers[0].Vendor,
+		"the legacy singular field must keep decoding")
+	assert.Equal(t, []string{"openai", "anthropic"}, router.cfg.Providers[1].Vendors,
+		"the multi-vendor field must decode both supported surfaces")
+}
+
 func TestRouter_FactoryAcceptsEmptyShapes(t *testing.T) {
 	cases := [][]byte{nil, []byte(""), []byte(" "), []byte("null"), []byte("{}"), []byte("[]")}
 	for _, raw := range cases {