)

## Problem

`SimplifyAggGroupBy` simplified `GROUP BY f(x)` to `GROUP BY x` without
verifying that `f(x)` is injective (one-to-one). This caused wrong
results:

| Expression | Why wrong |
|---|---|
| `a * 0` / `0 * a` | always evaluates to 0 — all rows fall into one
group |
| `0 / a` | always evaluates to 0 |
| `a / 0` | division by zero |
| `a + NULL` / `a * NULL` / ... | always evaluates to NULL |
| `a * 0.1` with float/double | precision loss may map different inputs
to same result |

## Fix

1. **`isBinaryArithmeticSlot`**: restructured to separate slot-expr from
literal,
then validate each independently. Float/double check runs early, before
   slot extraction.

2. **New `checkLiteral(expr, literal)`**: rejects NULL literal and
   Multiply/Divide by zero.

3. **New `canExtractSlot(expr)`**: replaces the old unconditional
`extractSlotOrCastOnSlot` — only accepts bare `Slot` or implicit
lossless
   widening casts (integral→integral, float→double, integral→decimal,
decimal→decimal). Range and scale are compared directly for correctness.

## Changes

- `SimplifyAggGroupBy.java`: +80 lines, rewritten core logic
- `ExpressionUtils.java`: -35 lines, removed unused `isSlotOrCastOnSlot`
/
  `extractSlotOrCastOnSlot`
- `SimplifyAggGroupByTest.java`: +216 lines, 25 tests covering all new
paths

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>