docs: set 1.15.0 release date and note Snyk test fixes in changelog

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Author: reeshika-h

changelog.md

@@ -2,9 +2,10 @@
 
 ## v1.15.0
 
-### Oct 21, 2026
+### Nov 02, 2026
 
 - Feat: Added the opt-in `Contentstack.Builder.setSerializeNulls(boolean)` option. When enabled, `null` values in request bodies are sent to the API instead of being dropped, so a field's value (for example a Number field on a localized entry) can be cleared. Off by default; existing behavior is unchanged.
+- Tests: Replaced hardcoded usernames and API keys in `LoginDetailTest` and `TestClient` with generated values to resolve Snyk Code findings (DX-22133, DX-22134). No change to the published artifact.
 
 ## v1.14.1