docs: update changelog for username functionality (#274)

Author: asajjad2

src/common/changelog.d/20250620_121737_areebsajjad21_7460_update_changelog_for_username_functionality.md

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+<!--
+A new scriv changelog fragment.
+
+Uncomment the section that is right (remove the HTML comment wrapper).
+For top level release notes, leave all the headers commented out.
+-->
+
+<!--
+### Removed
+
+- A bullet item for the Removed category.
+
+-->
+
+### Added
+
+username generation functionality:
+
+- usernameify() - Generate usernames from full names with email fallback
+- create_user_with_generated_username() - Create users with automatic username collision handling
+- _find_available_username() - Handle username conflicts by appending numeric suffixes
+- _reformat_for_username() - Clean and format strings for username use
+- is_duplicate_username_error() - Detect database integrity errors for username collisions
+
+<!--
+### Changed
+
+- A bullet item for the Changed category.
+
+-->
+<!--
+### Deprecated
+
+- A bullet item for the Deprecated category.
+
+-->
+<!--
+### Fixed
+
+- A bullet item for the Fixed category.
+
+-->
+<!--
+### Security
+
+- A bullet item for the Security category.
+
+-->