[Feature]: Publish a2a-cpp to the Microsoft vcpkg registry

[MisterVVP]

Proposal

Publish a2a-cpp to the Microsoft vcpkg registry

Goal

Publish a2a-cpp as an official port in the public microsoft/vcpkg registry.

The repository already has a working local vcpkg overlay port for a2a-cpp. The next step is to convert that validated overlay port into a registry-ready vcpkg port, compute the release archive hash, update the vcpkg version database, and open a PR to microsoft/vcpkg.

Acceptance criteria

• a2a-cpp:x64-windows installs successfully from the registry-style port.
• a2a-cpp[postgres-store]:x64-windows installs successfully.
• The port uses vcpkg_from_github with the v0.2.0 release archive.
• The port has a real SHA512, not 0.
• vcpkg format-manifest ports/a2a-cpp/vcpkg.json succeeds.
• vcpkg x-add-version a2a-cpp updates the version database.
• The final vcpkg diff contains only the intended port and version database files.
• A PR is opened against microsoft/vcpkg.

[MisterVVP]

This may take longer due to public vcpkg registry requirements here