Prep for beta release of System.ClientModel (Azure#49453)

* prep for beta release of scm

* fix date

* Update changelog for ModelReaderWriter fix

---------

Co-authored-by: m-nash <[email protected]>

Author: m-redding

sdk/core/System.ClientModel/CHANGELOG.md

@@ -1,14 +1,10 @@
 # Release History
 
-## 1.4.0-beta.3 (Unreleased)
-
-### Features Added
-
-### Breaking Changes
+## 1.4.0-beta.3 (2025-04-16)
 
 ### Bugs Fixed
 
-### Other Changes
+- System.ClientModel.SourceGeneration adds the Default property even when no type builders exist.
 
 ## 1.4.0-beta.2 (2025-04-14)