fix(driver-mongodb): take a structured GroupByNode, and answer count/count_distinct like every other backend

.changeset/mongodb-structured-groupby-and-count-distinct.md

@@ -0,0 +1,66 @@
+---
+"@objectstack/driver-mongodb": patch
+"@objectstack/spec": patch
+---
+
+fix(driver-mongodb): take a structured `GroupByNode`, and answer `count` /
+`count_distinct` the way every other backend does (#6850, part of #6814)
+
+`driver-mongodb` is now enrolled in the shared `AGGREGATION_CASES` standard
+(`@objectstack/spec/data`), and clearing that cell fixed three divergences — all
+three of the kind that ANSWER rather than fail, which is why none of them ever
+surfaced as an error.
+
+**1. A structured `groupBy` node had no lowering at all (#6850).**
+`GroupByNodeSchema` declares a union: a bare field name, or
+`{ field, dateGranularity?, alias? }`. The pipeline builder annotated `groupBy`
+as `string[]` and did `groupId[field] = '$' + field`, so a structured node — an
+object in that loop — stringified: the `$group._id` key became the literal
+`"[object Object]"` and its value the field path `"$[object Object]"`, which
+matches nothing. The aggregation did not refuse and did not throw. It returned
+rows grouped by a nonexistent path, under a column named `[object Object]`.
+`MongoDBDriver.aggregate` passed the value through an `any` cast, which is why
+the declared union never met that annotation at `tsc`.
+
+Both sides now spell the declared type, so the next drift between them is a
+compile error. The `$group._id` keys on `alias ?? field` and its value is the
+FIELD path — the projected column is renamed, the grouping does not move, which
+is the rule #6401 converged the three SQL faces onto and the one
+`in-memory-aggregation.ts` has always applied. The bare-string spelling emits
+exactly what it emitted before.
+
+**2. `count_distinct` counted NULL as a distinct value (#6814).** The lowering
+collects a `$addToSet` and sizes it; `$addToSet` keeps an explicit `null`, so a
+nullable column answered one HIGHER than `COUNT(DISTINCT col)` — 3 where the
+standard says 2. The sizing now excludes null, which is what
+`COUNT(DISTINCT col)` computes on SQLite, PostgreSQL and MySQL alike and what
+`objectql`'s in-memory fallback already computed.
+
+**3. `count(col)` counted ROWS, not values.** Measured while writing the suite
+and named by neither issue: the `count` arm ignored `field` entirely and emitted
+`{ $sum: 1 }` for both spellings, so `count(stage)` came back 6 — the answer
+`count(*)` already has — where the standard says 4. `count(col)` now counts
+non-null values, and a missing field is counted as null, the SQL reading.
+
+**A `dateGranularity` node is now REFUSED rather than silently ignored**, with
+`NOT_IMPLEMENTED` / 501 in the ADR-0112 envelope — the same refusal, first
+sentence for first sentence, that `driver-sql` and `driver-turso`'s remote
+transport give for a granularity they cannot bucket (#6212). This driver
+publishes no `supports.queryDateGranularity`, so the engine buckets every
+granularity in memory and never pushes a bucketed node down; the refusal fires
+only for a caller that reached the builder directly, which previously got a
+`"[object Object]"` grouping instead. A native `$dateTrunc` lowering is
+buildable and is not ruled out — it needs the engine's bucket LABELS, a
+published capability record and `date-bucket-parity.test.ts`, so it is its own
+change. A `groupBy` entry that is neither half of the union is refused with
+`INVALID_QUERY` / 400.
+
+The suite that holds all of this is server-free (`mongodb-aggregation-
+translation.test.ts`): this package's real-mongod suites are opt-in since #5517,
+so it drives the EMITTED pipeline through a strict in-process evaluator that
+refuses every shape it does not model. It holds the lowering to the shared
+table; it does not answer "does MongoDB agree?", which is a real-mongod half's
+question and is recorded as still open on #6814.
+
+`driver-memory`'s half of #6814 is untouched — it remains under the #5499
+investment freeze, and its `AGGREGATION_CASES` DEBT row stands.