Resolve recursive language definations causing incorrect x-ms-pageabl…

…e methods (#1303)

# Description
This is a second attempt at #1281, with a bit of a deeper dig into how we ran into the situtation.

Pavel pointed out during a discussion that we really shouldn't be seeing any of these "extra" methods at the level #1281 was filtering them out. 

After applying industrial strength bisect I was able to come up with this test case:  
```yaml
!CodeModel 
operationGroups:
  - !OperationGroup 
    operations:
      - !Operation 
        language: !Languages 
          default:
            paging:
              nextLinkName: nextLink
      - !Operation 
        language: !Languages 
          default:
            paging:
              nextLinkName: nextLink
              nextLinkOperation: !Operation &ref_17
                language: !Languages 
                  default:
                    paging:
                      nextLinkName: nextLink
                      nextLinkOperation: *ref_17
      - *ref_17

```

In it this code:

```csharp
            foreach (var op in v.OperationGroups.First().Operations)
            {
                var paging = op.Language.Default.Paging;
                bool hasNextOperation = paging?.NextLinkOperation != null;
```
would return true for the second and third operation in my simplified model but the third (*ref_17) would differ in the "full" autorest.csharp model.

The culprit was surprisingly:

```
-    internal partial class Language : IDictionary<string, object>
+    internal partial class Language
```

For some, honestly unknown, reason YamlDotNet would not handle this case correctly as long as we implemented the dictionary interface. I decided against deep diving into that 3rd party dependency to understand this further. This already cost too many days. I did try bumping to latest, which failed for unrelated reasons but did not change this behavior.

Removing the IDictionary interface required adding a few fields (header, summary) that were being indirectly handled, and may require more when I test this with azure-sdk-for-net this afternoon.

Test case included for #1056 in particular.

The removals in the paging test suites are expected.

src/AutoRest.CSharp/Common/Input/CodeModelPartials.cs

@@ -176,7 +176,7 @@ internal partial class Paging
     }
 
     /// <summary>language metadata specific to schema instances</summary>
-    internal partial class Language : IDictionary<string, object>
+    internal partial class Language
     {
         /// <summary>namespace of the implementation of this item</summary>
         [YamlMember(Alias = "namespace")]
@@ -198,63 +198,11 @@ internal partial class Language : IDictionary<string, object>
         [YamlMember(Alias = "paging")]
         public Paging? Paging { get; set; }
 
-        [YamlIgnore]
-        public IDictionary<string, object> AdditionalProperties = new Dictionary<string, object>();
+        [YamlMember(Alias = "header")]
+        public string? Header { get; set; }
 
-        private readonly Dictionary<string, object> _dictionary = new Dictionary<string, object>();
-        private static readonly Dictionary<string, PropertyInfo> DeserializableProperties = typeof(Language).GetDeserializableProperties();
-
-        // Workaround for mapping properties from the dictionary entries
-        private void AddAndMap(string key, object value)
-        {
-            _dictionary.Add(key, value);
-
-            if (DeserializableProperties.ContainsKey(key))
-            {
-                var propInfo = DeserializableProperties[key];
-                propInfo.SetValue(this, propInfo.DeserializeDictionary(value));
-                return;
-            }
-
-            AdditionalProperties.Add(key, value);
-        }
-
-        public IEnumerator<KeyValuePair<string, object>> GetEnumerator() => _dictionary.GetEnumerator();
-        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
-
-        public void Add(KeyValuePair<string, object> item) => AddAndMap(item.Key, item.Value);
-        public void Clear() => _dictionary.Clear();
-        public bool Contains(KeyValuePair<string, object> item) => _dictionary.ContainsKey(item.Key);
-        public void CopyTo(KeyValuePair<string, object>[] array, int arrayIndex)
-        {
-            foreach (var item in _dictionary)
-            {
-                array[arrayIndex++] = item;
-            }
-        }
-        public bool Remove(KeyValuePair<string, object> item) => _dictionary.Remove(item.Key);
-
-        public int Count => _dictionary.Count;
-        public bool IsReadOnly => false;
-        public void Add(string key, object value) => AddAndMap(key, value);
-        public bool ContainsKey(string key) => _dictionary.ContainsKey(key);
-        public bool Remove(string key) => _dictionary.Remove(key);
-
-        public bool TryGetValue(string key, out object value)
-        {
-            var result = _dictionary.TryGetValue(key, out var outValue);
-            value = outValue ?? String.Empty;
-            return result;
-        }
-
-        public object this[string key]
-        {
-            get => _dictionary[key];
-            set => AddAndMap(key, value);
-        }
-
-        public ICollection<string> Keys => _dictionary.Keys;
-        public ICollection<object> Values => _dictionary.Values;
+        [YamlMember(Alias = "summary")]
+        public string? Summary { get; set; }
     }
 
     internal partial class NoAuthSecurity : SecurityScheme

test/AutoRest.TestServer.Tests/paging.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Reflection;
 using System.Threading.Tasks;
 using AutoRest.TestServer.Tests.Infrastructure;
 using Azure;
@@ -846,5 +847,22 @@ public void PagingModelsAreHidden()
         {
             Assert.IsFalse(typeof(ProductResult).IsPublic);
         }
+
+        [Test]
+        public void RestClientDroppedMethods()
+        {
+            var client = typeof(PagingRestClient);
+            Assert.IsNull(client.GetMethod ("CreateNextFragmentNextPageRequest", BindingFlags.Instance | BindingFlags.NonPublic), "CreateNextFragmentNextPageRequest should not be generated");
+            Assert.IsNull(client.GetMethod ("NextFragmentNextPageAsync", BindingFlags.Instance | BindingFlags.NonPublic), "NextFragmentNextPageAsync should not be generated");
+            Assert.IsNull(client.GetMethod ("NextFragmentNextPage", BindingFlags.Instance | BindingFlags.NonPublic), "NextFragmentNextPage should not be generated");
+        }
+
+        [Test]
+        public void ClientDroppedMethods()
+        {
+            var client = typeof(PagingClient);
+            Assert.IsNull(client.GetMethod ("NextFragmentAsync", BindingFlags.Instance | BindingFlags.NonPublic), "NextFragmentAsync should not be generated");
+            Assert.IsNull(client.GetMethod ("NextFragment", BindingFlags.Instance | BindingFlags.NonPublic), "NextFragment should not be generated");
+        }
     }
 }