feat: add Entry Variant support and integration tests

Add EntryVariant model and Entry.Variant() for create/fetch/find/update/delete
against entry variant endpoints.

Extend publish flow with PublishVariant, PublishVariantRules, and serialization
in PublishUnpublishDetails / PublishUnpublishService; update unit tests for
variant publish JSON.

Add Contentstack021_EntryVariantTest: Product Banner lifecycle (setup, create,
fetch, publish, delete) with config-based stack API key, and negative cases
for invalid entry/variant, publish, environments, and unlinked content types.

Update CHANGELOG for Entry Variant support.

Author: OMpawar-21

CHANGELOG.md

@@ -1,5 +1,13 @@
 # Changelog
 
+## [v0.8.0](https://github.com/contentstack/contentstack-management-dotnet/tree/v0.8.0)
+ - Feat
+   - **Entry Variant support**
+     - `EntryVariant` model for create, fetch, find, update, and delete on entry variant endpoints
+     - `Entry.Variant(uid)` to access variant operations for a given entry
+     - Publish with variants: `PublishVariant`, `PublishVariantRules`, and `Variants` / `VariantRules` on `PublishUnpublishDetails`; serialization updated in `PublishUnpublishService`
+     - Unit tests for `EntryVariant` and publish payload serialization; integration tests (`Contentstack021_EntryVariantTest`) for Product Banner lifecycle and negative cases
+
 ## [v0.7.0](https://github.com/contentstack/contentstack-management-dotnet/tree/v0.7.0)
  - Feat
    - **Bulk publish/unpublish: query parameters (DX-3233)**

Contentstack.Management.Core.Tests/IntegrationTest/Contentstack015_BulkOperationTest.cs

@@ -1789,6 +1789,9 @@ public class SimpleEntry : IEntry
         {
             [JsonProperty(propertyName: "title")]
             public string Title { get; set; }
+
+            [JsonProperty(propertyName: "_variant")]
+            public object Variant { get; set; }
         }
 
         public class EntryInfo