Fix NaturalKeySource discovery when methods are on the projection class

[jeremydmiller]

Summary

When [NaturalKeySource] methods are defined on the projection class (e.g., SingleStreamProjection<TDoc, TId>) rather than the aggregate type itself, the natural key discovery failed to find them. This caused FetchLatest and FetchForWriting with natural keys to return null.

Root Cause

discoverNaturalKey() only searched the aggregate type (typeof(TDoc)) for [NaturalKeySource] methods. When Create/Apply methods with the attribute were on the projection class, the EventMappings list remained empty and the NaturalKeyProjection never populated the lookup table.

Fix (in JasperFx.Events)

1. discoverNaturalKey() now also searches GetType() (the projection class) for [NaturalKeySource] methods
2. For aggregate instance methods: preserve the original working pattern (create instance, call method, read property)
3. For projection static methods: find a matching property on the event data type by natural key type (since calling the static method may crash when it needs IEvent.StreamKey)

Test

New test natural_key_source_on_projection_class_with_string_streams with BookListing aggregate + Isbn natural key where all [NaturalKeySource] methods are on the projection class.

Depends on JasperFx/jasperfx@b5aba64 (pushed to main)

Test plan

• 28/28 natural key tests passing (27 existing + 1 new), zero regressions

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