Emit a deprecation warning when serializing an invalid Date

[Copilot]

new Date(undefined) (or any invalid Date) is silently serialized to "0NaN-NaN-NaNTNaN:NaN:NaN.NaN+NaN:NaN", a string Postgres cannot meaningfully parse, leading to confusing downstream errors instead of a clear signal at the source.

Changes

• packages/pg/lib/utils.js: prepareValue now emits a util.deprecate-based process warning (code PG_INVALID_DATE) when serializing an invalid Date, without changing the current serialized output, for backwards compatibility. The warning explains that this will become an error in the next major version of pg.
• packages/pg/test/unit/utils-tests.js: added a unit test asserting prepareValue emits the PG_INVALID_DATE warning and still serializes the invalid date to the existing (garbage) string.

const { Client } = require('pg')
const client = new Client()

// Still serializes to "0NaN-NaN-NaNTNaN:NaN:NaN.NaN+NaN:NaN" today,
// but now also emits a DeprecationWarning (code PG_INVALID_DATE)
await client.query('INSERT INTO events (occurred_at) VALUES ($1)', [new Date(undefined)])

• Fixes Do not serialize new Date(undefined) as "0NaN-NaN-NaNTNaN:NaN:NaN.NaN+NaN:NaN" #3318