Debug bls12 vcpkg NuGet cache access

[huangminghuang]

Why

This diagnostic PR checks whether Wire-Network/wire-cdt can access and publish the bls12-381_x64-linux-release vcpkg NuGet cache package using its normal workflow GITHUB_TOKEN.

This helps compare wire-cdt package access with the wire-sysio cache failures, where NuGet returned 403 Forbidden for the same package id.

What

• Adds a temporary diagnostic workflow only.
• Probes the GitHub Packages NuGet metadata endpoint for bls12-381_x64-linux-release.
• Runs vcpkg install from a temporary manifest containing only bls12-381.
• Uses VCPKG_BINARY_SOURCES=clear;nuget,https://nuget.pkg.github.com/Wire-Network/index.json,readwrite.
• Does not build CDT and does not run CDT tests.

Expected Signal

• Metadata probe HTTP=200 means the wire-cdt token can see the package metadata.
• Metadata probe HTTP=403 means the package exists but is inaccessible to the wire-cdt token.
• vcpkg restore/push logs should show whether wire-cdt can restore or publish bls12-381_x64-linux-release.

This PR is intended to be closed after collecting the CI logs.