feat: pgerr error inspection package and DB.InTx method

[klaidliadon]

Two additive changes for the dbkit lift happening downstream. Both are surfaces that consumers were about to re-implement in every project.

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pgerr sub-package

Postgres error inspection helpers built on pgconn.PgError. Sits on top of github.com/jackc/pgerrcode for SQLSTATE constants; pgerr's own surface is just the inspection helpers that operate on error rather than raw code strings.

func IsErrorNoRows(err error) bool                  // pgx.ErrNoRows | sql.ErrNoRows
func IsUniqueViolation(err error) (string, bool)    // comma-ok: returns constraint name when SQLSTATE 23505
func IsFatal(err error) bool                        // class-based retry/alert gate

IsFatal treats SQLSTATE classes 00 (success), 01 (warning), 02 (no data), and 23 (integrity constraint violation) as non-fatal; every other class is fatal. Non-PgError errors return false. Class derivation uses the first two characters of the SQLSTATE code (not pgerrcode's per-class membership helpers), so any future code added within those classes is automatically covered.

DB.InTx(tx) *DB

func (d *DB) InTx(tx pgx.Tx) *DB

Returns a new *DB sharing Conn and SQL with the parent but routing queries through tx. Complements the existing TxQuery(tx) *Querier so callers can pass a tx-scoped *DB into anything that takes *pgkit.DB without rewriting parameter types. Pattern that every binary doing transactional flows ends up needing.

WithTx was considered for the name; rejected because pgkit.WithTx(ctx, tx) context.Context already exists at the package level and the cognitive overlap is bad. InTx reads cleanly at the call site.

Test plan

go test ./... excluding ./tests/... (DB-dependent integration tests) — 24 subtests across pgerr and pgkit root, all green.

go vet ./... clean, gofmt -l . clean, go build ./... clean.

[VojtechVitek]

I think it's OK to add github.com/jackc/pgerrcode to go.mod dependencies. We rely on jack's pgx and other packages already.

Check this out - the repo is just this file: https://github.com/jackc/pgerrcode/blob/master/errcode.go