postgres: emit full leading-zero groups when binary-decoding sub-1e-8 NUMERIC (#31211)

The Postgres binary NUMERIC decoder collapsed Postgres' two
`get_str_from_var` counters — a base-10000 digit index and a
decimal-place position — into a single index that advanced by 4 but also
gated the leading-zero region. For `weight <= -3` (values below 1e-8
with no integer part) this walked the leading "0000" groups 4× too fast
and dropped one or more of them, shifting every significant digit left
by 4 decimals per dropped group (e.g. `1e-9` decoded as `0.000010000`).
Values ≥ 1e-8, and any value with an integer part, were unaffected;
text-protocol decoding was always correct.

Split the two counters back apart, mirroring `get_str_from_var`: one
walks the base-10000 digits, the other counts emitted decimal places and
drives truncation to `dscale`. The leading-zero region now emits a full
"0000" group per missing digit position.

Adds a mock-Postgres-server test covering a matrix of sub-1e-8 and
boundary values. (Not a port regression — the Zig decoder has the same
defect, but Node/psql are correct.)