CHANGELOG: add entry for edge-case validation tests (#33)

Author: dannywillems

CHANGELOG.md

@@ -15,6 +15,12 @@ and this project adheres to
 - Tests now import from `l9format` package directly instead of
   `l9format.l9format` ([e8aef2e], [#21])
 
+### Added
+
+- Add 34 edge-case and validation tests covering missing fields, null values,
+  empty strings, boundary integers, malformed datetimes/decimals, and nested
+  validation ([0247fd9], [#33])
+
 ### Infrastructure
 
 - CI: enforce CHANGELOG.md changes are in dedicated commits
@@ -137,6 +143,7 @@ and this project adheres to
 
 <!-- Commit links -->
 
+[0247fd9]: https://github.com/LeakIX/l9format-python/commit/0247fd9
 [d30efd2]: https://github.com/LeakIX/l9format-python/commit/d30efd2
 [1dcfbef]: https://github.com/LeakIX/l9format-python/commit/1dcfbef
 [e8aef2e]: https://github.com/LeakIX/l9format-python/commit/e8aef2e
@@ -199,4 +206,5 @@ and this project adheres to
 [#16]: https://github.com/LeakIX/l9format-python/pull/16
 [#18]: https://github.com/LeakIX/l9format-python/pull/18
 [#21]: https://github.com/LeakIX/l9format-python/issues/21
+[#33]: https://github.com/LeakIX/l9format-python/issues/33
 [#35]: https://github.com/LeakIX/l9format-python/issues/35