update changelogs

Author: patrickthornton

fern/products/sdks/overview/csharp/changelog/2026-01-29.mdx

@@ -1,3 +1,7 @@
+## 2.19.7
+**`(fix):`** Top-level clients now use interface types for sub-clients instead of concrete types.
+
+
 ## 2.19.6
 **`(fix):`** Fix wire test generation to preserve string field values that contain datetime-like strings.