Translate the LINQ operators that materialise a sequence

[joadan]

ToDictionary, ToLookup and GroupBy were classified Unsupported, so a client writing

e.Nodes.ToDictionary(n => n.ResourceReference, n => n, StringComparer.OrdinalIgnoreCase)

got "the LINQ operator 'ToDictionary' cannot be translated to a GraphQL selection" instead of a query. Nothing about them is untranslatable: the query tree only records which fields are mentioned and where they hang, and their lambdas read fields of a single sequence — the pass-through shape. The keying itself happens on the client after the response is deserialized.

What changed

• LinqOperator — added the single-sequence operators to PassThroughOperators: ToDictionary, ToLookup, GroupBy, plus the ones the list had never caught up with (Append, Contains, CountBy, Index, Prepend, Shuffle, TryGetNonEnumeratedCount). LinqOperatorKind now says what Unsupported actually means: operators that combine several sequences (Concat, Join, Zip) or fold into an accumulator (Aggregate).
• QueryExpressionVisitor — pass-through lambdas bind their later parameters too when the parameter is a sequence of the element type, so GroupBy's result selector (key, orders) => … can select through orders instead of reporting it as unbound.
• Error message — the unsupported-operator message now names the way out, since the operator can almost always be applied to the result of ExecuteAsync.
• GraphQueryTranslationException — translation failures were the one failure category with no named type and no structured context; they now throw this, carrying the offending Expression and the operator/member name. It derives from NotSupportedException, which is what the parser threw before, so existing catch clauses keep working.

Why the list stays an allow-list

Classifying by shape instead would remove the maintenance, but it changes how the code fails when it meets an operator we haven't anticipated. An unlisted operator throws today: loud, and one line to fix. Guessing pass-through would instead emit a query missing fields, which surfaces as nulls and defaults in a deserialized object with no error anywhere — the failure mode the parser rewrite set out to eliminate. It also wouldn't remove the name knowledge entirely, since Select/SelectMany must still be recognized to move the selection and the combining operators still need explicit rejection.

Tests

Four parser cases in ExpressionParserTests (ToDictionary with an element selector, with a comparer, GroupBy, GroupBy with a result selector) and two end-to-end cases in QueryOperatorTests. Both UnsupportedOperator_IsReported tests used GroupBy as their example and now use Concat; they assert the hint and MemberName, and the parser one pins that the exception is still assignable to NotSupportedException. Full CI suite: 166 passed. No generator or template changes, so the generated-clients drift check is unaffected.