fix(pg): make prepared-statement caches prototypeless

[nirmal-shaji]

Problem

A prepared statement whose name collides with an Object.prototype key — most obviously constructor, but also hasOwnProperty, toString, __proto__, etc. — is mishandled. The client believes the statement has already been prepared when it has not, so it skips the Parse message and sends a Bind for a statement the backend has never seen, producing a protocol error rather than running the query.

Cause

Connection tracks which named statements have been prepared using plain object literals:

this.parsedStatements = {}
this.submittedNamedStatements = {}

The native Client does the same with this.namedQueries = {}. These caches are keyed by user-supplied statement names, and Query#hasBeenParsed / Query#submit test membership with a simple truthy lookup:

connection.parsedStatements[this.name] || connection.submittedNamedStatements[this.name]

Because the objects inherit from Object.prototype, parsedStatements['constructor'] resolves to Object.prototype.constructor (truthy) on a brand-new connection, so Parse is never sent.

Fix

Create the three caches with Object.create(null) so lookups only ever reflect statements that were actually prepared. This matches the request in #3625 (make parsedStatements prototypeless, alongside namedQueries on the native client); submittedNamedStatements is fixed too since it is consulted in the same || expression and shares the flaw. No API surface changes.

Testing

Added a pure-Node unit test (no live Postgres) at packages/pg/test/unit/connection/prepared-statement-name-tests.js, mirroring the existing test/unit/connection tests and using the in-memory stream helper. It asserts that:

• a fresh connection reports no prepared statements for Object.prototype names;
• a statement named constructor sends a Parse before its Bind;
• hasBeenParsed is false for unprepared prototype-named statements.

The test fails on master and passes with this change. The full packages/pg unit suite (make test-unit, run file-by-file with node) remains green.

Targeting the default branch (master); the issue is tagged toward the pg@9.0 milestone — happy to retarget if a release branch is preferred.

Fixes #3625

[charmander]

#3732 (comment)