Fix #4680 — upcaster shadowed when source type is typed-appended in same DocumentStore

[jeremydmiller]

Closes #4680.

When Upcast<TOld, TNew>(...) is registered AND a TOld instance is appended (typed) into the same DocumentStore, reading back returned TOld instead of TNew. Reading from a different store instance worked, which the issue reporter narrowed to "appending the typed source event registers a concrete EventMapping for TOld that shadows the upcaster on read."

Mechanism

Step	What happens
1	Upcast(typeof(TNew), oldEventName, transformation) builds an EventMapping<TNew> keyed under oldEventName in _byEventName. Its ReadEventData runs the JsonTransformation to produce TNew.
2	Typed Append(TOld) flows through EventMappingFor(typeof(TOld)), whose OnMissing factory lazily creates an EventMapping<TOld>. The new mapping lives in the registry alongside the upcaster mapping.
3	On EventDocumentStorage.Resolve / ResolveAsync, the by-name lookup returns the upcaster (good), but then the dotnet_type column fires the existing alt-mapping swap: TryGetRegisteredMappingForDotNetTypeName(dotnetTypeName) finds the EventMapping<TOld> the typed Append created, replaces the upcaster, and the deserializer materialises TOld -- bypassing the upcaster entirely.

The alt-mapping swap is correct for its original purpose: same EventTypeName, multiple CLR types, NO upcaster registered (e.g. moved-namespace renames). The bug is that the swap fires unconditionally even when the by-name mapping is an authoritative upcaster.

Fix

New EventMapping.IsUpcastTarget flag, set in EventGraph.Upcast(...). EventDocumentStorage.Resolve / ResolveAsync skip the dotnet_type alt-mapping swap when the by-name mapping is an upcast target. The swap remains for the non-upcaster case it was originally added for.

Tests

src/EventSourcingTests/SchemaChange/Bug_4680_upcaster_shadowed_by_typed_append.cs:

• same_store_typed_append_is_upcast_on_read -- the headline repro. Failing before this commit, passing after.
• alt_mapping_swap_still_works_when_no_upcaster_registered -- regression pin for the alt-mapping swap's legitimate use case. Two CLR types share \"shared_event_name\" with no upcaster; the dotnet_type column should still steer the resolver to the appended type's mapping. Pinning this protects against the fix over-rotating.

Project	Result
EventSourcingTests	1363 / 1370 (7 unrelated skips) on net9.0

Including the 2 new bug tests; no other regressions.

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