fix(pg-cloudflare): prevent Client.end() from hanging

[pratik-desgn]

Fixes #3152

Summary

• complete Cloudflare shutdown when Socket.close() resolves but Socket.closed never settles
• emit close once, with an identity guard preventing a duplicate event from the normal closed handler
• add a regression test for a socket that never settles its closed promise

The callback overload used by Connection.end() is already present in the current base branch via #3747.

Validation

• yarn lint
• yarn workspace pg-cloudflare build
• yarn workspace pg-esm-test test --test-name-pattern=pg-cloudflare (12 passed)
• GitHub CI: lint and all 11 Node/PostgreSQL matrix jobs passed

The PR is limited to pg-cloudflare shutdown behavior and its focused ESM test.