[sagarpjpt]

Fixes #3850.

pg already calls cork() and uncork() around a parameterized query when the socket supports them. CloudflareSocket lacked those methods, so Parse, Bind, Describe, Execute, and Sync went to its underlying writer in five separate calls.

This buffers writes until the last uncork() and sends their bytes in order as one write. Buffered callbacks receive the write result in order. end() flushes before closing, and destroy() discards unsent bytes and completes their callbacks with the destruction reason.

Validation

• The five new regression tests fail on master and pass with the fix. The protocol test checks that five writes become one with identical bytes.
• In local workerd against PGlite over TCP, parameterized queries used one writer call each. The same connection handled another query after a rejected parameter value. The existing Workers test also passed.
• pg unit suite, 29 ESM tests, TypeScript build, lint, and webpack, Rollup, Vite, and esbuild checks passed.

Production latency was not measured locally. Prepared with AI assistance.