Changed Error Message if Entity is not configured in dab-config json (#2186)

## Why make this change?

As mentioned here #887, if entity is not configured in DAB Config JSON
file, it throws a generic exception i.e. keyNotFoundException.

As part of this PR, changed this message to make it clearer.

## What is this change?

If Entity is not configured then DAB will throw
`System.ApplicationException` with error message as _"The entity
'<EntityName>' was not found in the dab-config json"_

## How was this tested?

- [ ] Integration Tests
- [ ] Unit Tests

## Sample Request(s)

- Example REST and/or GraphQL request to demonstrate modifications
- Example of CLI usage to demonstrate modifications

Author: sourabh1007

src/Config/ObjectModel/RuntimeEntities.cs

@@ -56,7 +56,21 @@ public bool ContainsKey(string key)
         return Entities.ContainsKey(key);
     }
 
-    public Entity this[string key] => Entities[key];
+    public Entity this[string key]
+    {
+        get
+        {
+            Entity? entity;
+            if (Entities.TryGetValue(key, out entity) && entity is not null)
+            {
+                return entity;
+            }
+            else
+            {
+                throw new ApplicationException($"The entity '{key}' was not found in the dab-config json");
+            }
+        }
+    }
 
     IEnumerator IEnumerable.GetEnumerator()
     {

src/Service.Tests/Configuration/ConfigurationTests.cs

@@ -19,6 +19,7 @@
 using System.Threading.Tasks;
 using System.Web;
 using Azure.DataApiBuilder.Config;
+using Azure.DataApiBuilder.Config.NamingPolicies;
 using Azure.DataApiBuilder.Config.ObjectModel;
 using Azure.DataApiBuilder.Core.AuthenticationHelpers;
 using Azure.DataApiBuilder.Core.Authorization;
@@ -2937,6 +2938,63 @@ public async Task TestEngineSupportViewsWithoutKeyFieldsInConfigForMsSQL()
             }
         }
 
+        [TestMethod, TestCategory(TestCategory.COSMOSDBNOSQL)]
+        public async Task TestErrorMessageWithoutKeyFieldsInConfig()
+        {
+            Dictionary<string, object> dbOptions = new();
+            HyphenatedNamingPolicy namingPolicy = new();
+
+            dbOptions.Add(namingPolicy.ConvertName(nameof(CosmosDbNoSQLDataSourceOptions.Database)), "graphqldb");
+            dbOptions.Add(namingPolicy.ConvertName(nameof(CosmosDbNoSQLDataSourceOptions.Container)), "dummy");
+            dbOptions.Add(namingPolicy.ConvertName(nameof(CosmosDbNoSQLDataSourceOptions.Schema)), "custom-schema.gql");
+            DataSource dataSource = new(DatabaseType.CosmosDB_NoSQL,
+                GetConnectionStringFromEnvironmentConfig(environment: TestCategory.COSMOSDBNOSQL), Options: dbOptions);
+            Entity entity = new(
+                Source: new("EntityName", EntitySourceType.Table, null, null),
+                Rest: new(Enabled: false),
+                GraphQL: new("", ""),
+                Permissions: new[] { GetMinimalPermissionConfig(AuthorizationResolver.ROLE_ANONYMOUS) },
+                Relationships: null,
+                Mappings: null
+            );
+
+            RuntimeConfig configuration = InitMinimalRuntimeConfig(dataSource, new(), new(), entity, "EntityName");
+
+            const string CUSTOM_CONFIG = "custom-config.json";
+
+            File.WriteAllText(
+                CUSTOM_CONFIG,
+                configuration.ToJson());
+
+            string[] args = new[]
+            {
+                $"--ConfigFileName={CUSTOM_CONFIG}"
+        };
+
+            using (TestServer server = new(Program.CreateWebHostBuilder(args)))
+            using (HttpClient client = server.CreateClient())
+            {
+                string query = @"{
+                    EntityName {
+                        items{
+                            id
+                            title
+                        }
+                    }
+                }";
+
+                object payload = new { query };
+
+                HttpRequestMessage graphQLRequest = new(HttpMethod.Post, "/graphql")
+                {
+                    Content = JsonContent.Create(payload)
+                };
+
+                ApplicationException ex = await Assert.ThrowsExceptionAsync<ApplicationException>(async () => await client.SendAsync(graphQLRequest));
+                Assert.AreEqual(ex.Message, "The entity 'Planet' was not found in the dab-config json");
+            }
+        }
+
         /// <summary>
         /// Tests that Startup.cs properly handles EasyAuth authentication configuration.
         /// AppService as Identity Provider while in Production mode will result in startup error.