Prepare Keyvault Keys beta release 4.12.0.b2

[Hariharan K (hari9-9)]

Description

Updating Changelog

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[azure-pipelines]

Azure Pipelines:
Successfully started running 1 pipeline(s).
35 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[Copilot]

Pull request overview

Updates the Key Vault Keys library changelog to finalize the 4.12.0-beta.2 entry for release, converting it from an unreleased placeholder to a dated release entry.

Changes:

• Marked 4.12.0-beta.2 as released on 2026-08-07 instead of (Unreleased).
• Removed empty section headers (Breaking Changes / Bugs Fixed / Other Changes) for 4.12.0-beta.2.