Fix multi-property record structs misclassified as strong-typed-id wrappers

[jeremydmiller]

Problem

When a document property's type is a multi-field record struct like Money(decimal Value, Guid CurrencyId), the LINQ resolver throws on nested member access:

'Marten.Exceptions.BadLinqExpressionException' ... 'Marten does not (yet) support member Money.Value'

Root cause

In ValueTypeIdGeneration.IsCandidate, the property filter only counts properties whose type is in DocumentMapping.ValidIdTypes (Guid/int/long/string). For a Money struct with Value: decimal and CurrencyId: Guid, it sees only the CurrencyId property — a single match. A static Money Zero(Guid) builder method then satisfies the 1-arg signature requirement, and the type is wrongly classified as a strong-typed-id wrapper.

Worse, the candidate check has a side effect: it calls

PostgresqlProvider.Instance.RegisterMapping(typeof(Money), "uuid", NpgsqlDbType.Uuid);

on the global Weasel singleton, poisoning the type map for the rest of the process. Subsequent LINQ resolution treats Money as a uuid scalar (SimpleCastMember) instead of a JSONB document (ChildDocument), so any x.MoneyProp.Amount > 0 style query throws.

Fix

Reject types whose canonical constructor takes more than one parameter before any further matching. A strong-typed-id wrapper takes exactly one argument by definition; a multi-property record struct's primary ctor (Money(decimal, Guid)) signals "data-carrying value object", not "id wrapper".

Single-arg strong-typed-ids — including those with additional convenience computed properties — continue to be detected normally. Verified: dotnet test src/ValueTypeTests passes 339/339.

Tests

Two regression tests in src/LinqTests/Bugs/:

1. multi_property_record_struct_does_not_pollute_global_pg_type_mapping — asserts PostgresqlProvider.Instance.GetDatabaseType(typeof(Money), ...) returns "jsonb" (the document-storage default) rather than "uuid".
2. linq_can_resolve_nested_member_access_on_multi_property_record_struct — end-to-end check that session.Query<Doc>().Where(x => x.Amount.Value > 0m && x.Amount.CurrencyId == Guid.Empty).ToCommand() no longer throws.

Each test uses its own unique record-struct type to avoid cross-test pollution of the process-wide PostgresqlProvider.Instance singleton.

Reported by

@mmidkiff, with full root-cause analysis pre-attached. Thanks!

Test plan

• New regression tests pass on net9.0
• dotnet test src/ValueTypeTests 339/339 — no regression on legit strong-typed-id detection
• Full CI green

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