Discover [NaturalKeySource] on static methods of self-aggregating aggregates

[jeremydmiller]

Addresses JasperFx/marten#4277.

Problem

For self-aggregating aggregates — records or classes where the projection and the aggregate are the same type — the `[NaturalKeySource]` attribute was silently ignored when placed on a static factory such as

public sealed record MyAggregate(Guid Id, [property: NaturalKey] MyKey Key)
{
    [NaturalKeySource]
    public static MyAggregate Create(MyCreated e) => new(e.Id, new MyKey(e.Key));
}

The discovery scan in `JasperFxAggregationProjectionBase.discoverNaturalKey` only picked up instance methods on the aggregate type, and a second scan only ran for static methods on a *different* projection class. When the projection and the aggregate were the same type the second scan was skipped, so the natural-key lookup row was never written and `FetchForWriting<TDoc, TKey>(key)` returned a null aggregate.

Fix

Two small changes:

1. `discoverNaturalKey` — include `BindingFlags.Static` alongside `BindingFlags.Instance` when scanning `docType`. Static methods on the aggregate are now candidates regardless of whether the projection is a separate class. Cleanly covers @mfmadsen's suggestion in the issue.
2. `buildExtractor` — add a branch for static methods on `docType` that return `docType`. Invoke the factory with the raw event payload and read the natural-key property off the returned aggregate, matching the semantics of the existing instance-method path.

Test coverage

New file `src/EventTests/Projections/NaturalKeySourceDiscoveryTests.cs` with three tests exercising the three discovery pathways directly against `NaturalKeyDefinition.EventMappings`:

• instance method on aggregate (pre-existing shape)
• static factory on self-aggregating aggregate (#4277)
• static method on a separate projection class (property-matching fallback)

All three pass on net8.0 / net9.0 / net10.0. Full `EventTests` suite (222 tests) stays green.

Version

`JasperFx.Events` bumped to 1.29.1 (patch bump — bug fix, no API changes).

Companion PR

A Marten-side companion PR adds an end-to-end regression test using the exact shape from the issue, matching `Bug_4197_fetch_for_writing_natural_key.cs`.

🤖 Generated with Claude Code