docs: update CHANGELOG for v1.7.3

Document the synced_from parsing fix that avoids undefined-key warnings when block_id is missing.

Author: brd6

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
+## 1.7.3 - 2026-02-19
+
+### Fixed
+
+- Avoid PHP warnings when Notion returns a `synced_from` object without a `block_id` key.
+
 ## 1.7.2 - 2025-10-23
 
 ### Fixed