chore: fixes grammar on 404 (envoyproxy#1413)

**Description**

This fixes the grammar in the English sentence in the response body to
404 cases.

---------

Signed-off-by: Takeshi Yoneda <[email protected]>
Signed-off-by: ayush <[email protected]>

Author: mathetake

cmd/aigw/testdata/translate_basic.out.yaml

@@ -182,8 +182,7 @@ metadata:
 spec:
   directResponse:
     body:
-      inline: No matching route found. It is likely that the model specified your
-        request is not configured in the Gateway.
+      inline: No matching route found. It is likely because the model specified in your request is not configured in the Gateway.
       type: null
     statusCode: 404
 ---

internal/controller/ai_gateway_route.go

@@ -146,7 +146,7 @@ func generateHTTPRouteFilters(aiGatewayRoute *aigv1a1.AIGatewayRoute) []*egv1a1.
 					Body: &egv1a1.CustomResponseBody{
 						Inline: ptr.To(
 							// "Likely" since the matching rule can be arbitrary, not necessarily matching on the model name.
-							`No matching route found. It is likely that the model specified your request is not configured in the Gateway.`,
+							`No matching route found. It is likely because the model specified in your request is not configured in the Gateway.`,
 						),
 					},
 				},

tests/e2e/testupstream_test.go

@@ -83,7 +83,7 @@ func TestWithTestUpstream(t *testing.T) {
 				name:            "openai",
 				modelName:       "non-existent-model",
 				expStatus:       404,
-				expResponseBody: `No matching route found. It is likely that the model specified your request is not configured in the Gateway.`,
+				expResponseBody: `No matching route found. It is likely because the model specified in your request is not configured in the Gateway.`,
 			},
 			{
 				name:               "openai-header-mutation",