ToSqlParam: support scaled Numeric (scale > 0) params — needs text-format bind

[StefanSteiner]

Summary

impl ToSqlParam for Numeric (added in #65) only supports scale = 0
(whole numbers). Scaled decimals (scale > 0) are rejected because Hyper
cannot read PG-binary NUMERIC parameters that require scale reconciliation.
This issue tracks lifting that limitation.

Background — why scaled Numeric params don't work today

query_params sends all parameters over the PostgreSQL extended-query
binary bind path (format code 1, hardcoded in
connection.rs:517-519).
For NUMERIC, the binary form is the PostgreSQL nbase-10000 packed-digit
wire format.

Empirically (probed against hyperd 0.0.25080):

Param	Result
Numeric scale 0 (e.g. 42)	✅ accepted
Numeric scale > 0 (e.g. 12345.6700)	❌ ERROR: Hyper cannot handle truncation when reading numerics. (0A000)
same, with explicit CAST($1 AS numeric(38,4))	❌ same error

The error is SQLSTATE 0A000 (feature_not_supported) and fires
regardless of the encoded dscale/weight or an explicit target type —
so it's a Hyper server limitation on reading binary NUMERIC params,
not a client-side encoding bug.

To avoid shipping something that looks supported but breaks on real
decimal data, #65 made Numeric::encode_param return an error (or the
impl rejects) when scale() > 0, failing fast with a clear message
instead of the cryptic server error.

Possible paths to full support

1. Text-format bind for Numeric. Allow per-parameter format codes so
   a Numeric param can travel as text (format code 0) instead of
   binary. Hyper parses '12345.6700'::numeric fine from text. This
   requires plumbing per-parameter format codes through
   start_execute_prepared / frontend::bind (currently
   param_formats is a uniform vec![1; n]). This is the most likely
   fix and is self-contained to the core client.
2. Upstream Hyper fix. If/when hyperd learns to read scaled binary
   NUMERIC params, the existing binary encoder can be extended to emit
   the full nbase-10000 format and the scale guard removed.
3. Hybrid. Keep binary for scale=0 (fast path) and fall back to text
   for scale>0.

Recommended: option 1 (per-parameter text format), since it also unlocks
clean text binding for any future type Hyper can't read in binary.

Acceptance criteria

• A Numeric with scale > 0 can be bound via query_params and
  round-trips correctly (e.g. 12345.6700 in → 12345.6700 out).
• The scale=0 fast path is preserved (no regression).
• A test covers several scales (0, 2, 4, 10) and negative values.

Related

• ToSqlParam coverage: Numeric, Interval, Geography, JSON #65 (ToSqlParam coverage) — added the scale=0-only Numeric impl and the
  scale>0 guard this issue removes.