#3166 fix: allow sqlc.arg/@name to fill gaps in numbered placeholder sequences

[nikolayk812]

Problem

Mixing sqlc.arg()/@name with explicit $N placeholders fails when the named construct occupies a lower-numbered position than the native placeholders:

-- fails with: could not determine data type of parameter $1
INSERT INTO users (a, b, c) VALUES (sqlc.arg(a), $2, $3);

The mirror case (named construct at the end) worked fine:
-- works
INSERT INTO users (a, b, c) VALUES ($1, $2, sqlc.arg(c));

This asymmetry was reported in #3166.

Root cause

validatePlaceholders ran before number() assigned numbers to sqlc.arg/@name/sqlc.slice occurrences. The gap check only considered native $N placeholders, so it saw {2, 3}, found a gap at 1, and fired the error — not knowing that sqlc.arg(a) was about to fill it.

#3166

e2e changes: sqlc.arg(named) now gets numbered before validation runs, so it fills $2 and the first real gap shifts from $2 to $3.