OXY-166: Add scalar aggregate functions (SUM / AVG / MIN / MAX) to oxygen-sql

[Kalin-Rudnicki]

OXY-166 — Scalar aggregate functions (SUM / AVG / MIN / MAX)

Adds SUM / AVG / MIN / MAX scalar aggregate support to the oxygen-sql query DSL, mirroring the existing COUNT path end-to-end (DSL → parse → fragment → decode). Scalar aggregation over the whole result set — no GROUP BY (that's OXY-100).

DSL

yield sum(p.age)   // Option[Long]
yield avg(p.age)   // Option[BigDecimal]
yield min(p.age)   // Option[Int]
yield max(p.age)   // Option[Int]

Nullability

SUM/AVG/MIN/MAX return SQL NULL over an empty result set, so all four decode to Option[_] (unlike COUNT, which is never null).

Type choices (documented in docs/sql/queries.md)

Follows Postgres' own widening:

Aggregate	Column	PG type	Scala result
sum	Short/Int	bigint	Option[Long]
sum	Long/BigDecimal	numeric	Option[BigDecimal]
sum	Float	real	Option[Float]
sum	Double	double precision	Option[Double]
avg	Short/Int/Long/BigDecimal	numeric	Option[BigDecimal]
avg	Float/Double	double precision	Option[Double]
min/max	any orderable A	same as A	Option[A]

The sum/avg widening is encoded by new SumType/AvgType type-classes, so the query's static type already reflects the widened result. The macro reads the widened Out type off the term and picks the matching optional decoder — the primitive int decoder matches on JDBC runtime class (java.lang.Integer), so decoding SUM(int) (a bigint/Long) without widening would fail; widening is required for correctness, not cosmetic.

Added a decode-only ResultDecoder.bigDecimal (Postgres numeric → java.math.BigDecimal). Intentionally no RowRepr[BigDecimal] / Column.Type.Numeric — that would ripple into the migration codecs; a BigDecimal table column is therefore still unsupported, but aggregate results decode to BigDecimal correctly.

Tests

• New CustomQuerySpec "scalar aggregates" test (real Postgres via TestContainers): SUM/AVG/MIN/MAX over a non-empty group and an empty group (NULL → None), plus SUM widening (→Long) and AVG type (→BigDecimal).
• Full sql-it suite: 55 passed / 0 failed.

See report/OXY-166.md for the full decision log and confidence score (9/10).

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