Publish the microsoft-webui Python package to PyPI

PR #453 adds the microsoft-webui Python package and builds all six cp311-abi3 wheels plus an sdist, attaching them to each GitHub Release. It deliberately does not publish them to PyPI.

Publishing was descoped because the ESRP and package-ownership policy for a Microsoft-owned PyPI project is unsettled. The build side is already done and validated, so this issue tracks only the publication step.

Prerequisites

• Decide the ESRP / signing policy for PyPI artifacts published under a Microsoft-owned account.
• Register and claim the microsoft-webui project on PyPI, and confirm the approved owner/co-owner accounts.
• Configure a PyPI Trusted Publisher for microsoft/webui, the publishing workflow file, and a protected pypi GitHub environment.
• Do the same on TestPyPI and perform a publish/install rehearsal before the first real release.

Implementation

• Add a workflow that publishes the exact release assets with OIDC Trusted Publishing, triggered by the GitHub Release published event.
• Validate the assets before upload rather than rebuilding them, so what ships is what the release pipeline already tested.
• Update docs/guide/integrations/python.md, docs/guide/installation.md, README.md, and crates/webui-python/README.md, which currently tell users to install from a release asset instead of pip install microsoft-webui.
• Update the Python distribution section of DESIGN.md, which records that WebUI does not publish to PyPI.

Notes

An earlier iteration of #453 contained a full OIDC publish workflow with provenance manifests and digest re-validation. It was removed rather than merged unused; the PR history has it if it is useful as a starting point.

Start by reviewing the prerequisites for ESRP, signing, ownership, and protected environments, then inspect the removed workflow in PR #453. Check the release assets and the TestPyPI rehearsal before adding publication support. Done means approved ownership and policy, a validated OIDC workflow, and updated guidance in the four listed documentation files plus DESIGN.md.