docs(metadata-protocol): fix stale where-slot arity note in ARRAY_VALUED_QUERY_SLOTS docblock (#8151)

The where row's last sentence described isFilterAST failing as if it were
the correct refusal (it produces malformedFilterArrayError, a 400 telling a
caller whose filters were each well-formed that their filter is malformed),
and was stale for the route it described: #7390 moved arity judgement for
this slot to the REST querystring ingress (assertFilterParamSuppliedOnce),
so a repeated ?filter= on GET /data/:object no longer reaches this block.
Also names the sharper half of #7390: ?filter=status&filter=%3D&filter=open
spells a valid AST and succeeds with a filter nobody expressed.

Comment-only, zero runtime change.

Fixes #8003


Claude-Session: https://claude.ai/code/session_01AmES43BMDg4bPrxTdi5q7t

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

Author: huangyiirene

packages/metadata-protocol/src/protocol.ts

@@ -1747,8 +1747,13 @@ const WIRE_DOLLAR_ALIASES: readonly (readonly [string, string])[] = [
  *                      and the array from either slot.
  *  - `where`         — a FILTER AST is an array (`['status','=','open']`), so a
  *                      blanket arity refusal here would reject the AST body form
- *                      outright. A repeated `?filter=` is still refused, one
- *                      block down, by `isFilterAST` failing to read it.
+ *                      outright. Arity for this slot is judged one layer up, at
+ *                      the REST querystring ingress (`assertFilterParamSuppliedOnce`,
+ *                      #7390) — the only layer that knows an array on this wire
+ *                      is a repetition and can be nothing else. A repeated
+ *                      `?filter=` never reaches this block on `GET /data/:object`;
+ *                      worse, `?filter=status&filter=%3D&filter=open` spells a
+ *                      valid AST and succeeds with a filter nobody expressed.
  *  - `groupBy`       — `z.array(GroupByNodeSchema)`.
  *  - `aggregations`  — `z.array(AggregationNodeSchema)`.
  *  - `joins` / `windowFunctions` — retired ARRAY keys (#4286). The tombstone,