Fix value type builder selection when a nullable sibling factory is present

[lucasbagrt]

Suggestion on a fix for issue #4288

Why

RegisterValueType selected the static builder method by parameter signature alone (FirstOrDefault over public
static methods with one parameter matching the wrapped value type). When a Vogen value object had a nullable sibling
factory — e.g. static T? FromNullable(string?) alongside static T From(string) — FirstOrDefault could pick the
nullable one. That builder's ReturnType is Nullable<T>, which then blew up inside ValueTypeInfo.CreateWrapper,
which assumes the builder returns T itself. The symptom reported in #4288 was a crash when generating DDL for any
computed index over the value type.

Changes

• src/Marten/StoreOptions.Identity.cs — RegisterValueType now collects every candidate with the correct parameter
  signature and prefers the one whose ReturnType == type, falling back to the first candidate. Preserves prior
  behaviour for value types that expose only a single matching builder.
• src/ValueTypeTests/Bugs/Bug_4288_value_type_with_nullable_factory.cs — regression test covering (1) DDL generation
  for a computed index over the value type and (2) round-trip + query using Bug4288Value, which exposes
  FromNullable.
• src/ValueTypeTests/registration.cs — new unit test picks_builder_whose_return_type_matches_value_type that pins
  the selection rule, plus a SpecialValueWithNullableSibling fixture (with From and FromNullable side by side).

Verification

• dotnet build src/Marten/Marten.csproj — succeeds.
• dotnet test src/ValueTypeTests/ValueTypeTests.csproj —
  registration.picks_builder_whose_return_type_matches_value_type (pure unit test, no Postgres dependency) passes;
  Bug_4288_value_type_with_nullable_factory
  (can_generate_ddl_for_index_over_value_type_with_nullable_sibling_factory and
  can_round_trip_and_query_value_type_with_nullable_sibling_factory) passes against a local Postgres; the existing
  registration suite stays green.
• Behaviour is unchanged for value types with only a single matching builder — the FirstOrDefault() fallback returns
  the same method the old code used to pick.