fix(pg-cloudflare): safely end already-closed sockets

[deepakganesh78]

Summary

• make CloudflareSocket.end() tolerate the underlying socket having already closed
• preserve the normal close call for open sockets and always invoke the end callback
• add pg-cloudflare unit-test support and regression coverage for the close-handler lifecycle

Root cause

_addClosedHandler() clears _cfSocket before emitting close. A later end()/destroy() still dereferenced _cfSocket after its write callback, causing a synchronous null.close() crash during client cleanup. The guarded close treats the already-closed state as complete without swallowing write errors or skipping the callback.

Related to #3695, which proposed the guard; this includes the regression test requested in that PR.

Tests

• corepack yarn workspace pg-cloudflare test — 2 passing
• corepack yarn build
• corepack yarn eslint --cache "packages/**/*.{js,ts,tsx}"

Fixes #3689