update changelogs

Author: patrickthornton

fern/products/sdks/overview/csharp/changelog/2026-01-28.mdx

@@ -0,0 +1,17 @@
+## 2.19.2
+**`(fix):`** Fixes missing references to Core utilities in interface files for the Optional wrapper.
+
+
+## 2.19.1
+**`(fix):`** Fix nullable element types in collections to correctly preserve nullable markers.
+
+Previously, nullable types nested inside collections would incorrectly strip the inner nullable marker:
+- `nullable<list<nullable<string>>>` incorrectly generated `IEnumerable<string>?`
+- `nullable<map<string, nullable<Address>>>` incorrectly generated `Dictionary<string, Address>?`
+
+Now generates correctly:
+- `nullable<list<nullable<string>>>` generates `IEnumerable<string?>?`
+- `nullable<map<string, nullable<Address>>>` generates `Dictionary<string, Address?>?`
+- `optional<map<string, nullable<string>>>` generates `Dictionary<string, string?>?`
+
+