Python: CHANGELOG Update with V1.0.0 Release

[giles17]

Motivation and Context

Update CHANGELOG.md file with V1.0.0 release

Description

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

Updates the Python changelog to document the 1.0.0 stable release, aligning release notes and compare links with the new tag.

Changes:

• Add a new ## [1.0.0] - 2026-04-02 release section with summarized Added/Changed/Removed/Fixed items.
• Update footer compare links so [Unreleased] now compares from python-1.0.0 and add a new [1.0.0] compare link.