test(package-feeds): assert the cargo branch of the package-manager wrapper #2357

Follow-up from the #2315 review.

wrapper_routes_only_its_own_process_and_carries_no_credential (src/compile/extensions/package_feeds.rs) asserts the npm, pip, and dotnet branches of the generated package-manager wrapper, but not cargo). That branch carries the most delicate settings:

• CARGO_HTTP_PROXY and CARGO_HTTP_CAINFO (process-scoped proxy and trust bundle);
• CARGO_CREDENTIAL_ALIAS_ADOAWPROXY plus CARGO_REGISTRY_GLOBAL_CREDENTIAL_PROVIDERS=adoawproxy.

Nothing checks that the Cargo token helper (PACKAGE_CARGO_TOKEN) renders only the non-secret ADO_MCP_TOKEN_SENTINEL.

Acceptance criteria

• Slice-assert the cargo) case for all four variables.
• Assert that the rendered token helper prints only the sentinel and references no credential variable.
• Cargo is not exercised by the live smoke (there is no Cargo runtime), so these unit assertions are its main guard.