Fix event adapter callback API not invoking adapters at runtime

Fixed a critical bug where event adapters configured via the NEW callback API
(journalBuilder callback parameter in WithJournal/WithJournalAndSnapshot) were
not being invoked at runtime, despite being correctly present in HOCON configuration.

Root Cause:
AkkaPersistenceJournalBuilder.Build() was creating a Config with
.WithFallback(Persistence.DefaultConfig()) which interfered with the main journal
configuration during HOCON merging, causing adapters to be registered but not invoked.

The Fix:
Removed the unnecessary .WithFallback(Persistence.DefaultConfig()) call in
AkkaPersistenceHostingExtensions.cs line 130. The adapter configuration blocks
(event-adapters and event-adapter-bindings) don't require fallback to default
persistence config and merge correctly with the journal config without it.

Impact:
This bug affected users with:
- Multiple journal configurations (e.g., default + sharding)
- NEW callback API: journalBuilder: journal => journal.AddWriteEventAdapter<...>()
- Explicit JournalOptions + WithJournalAndSnapshot pattern

This fix is especially critical now that the old Adapters property has been
deprecated in #665. All users must use the callback API which was broken.

Testing:
- Added comprehensive regression test suite (EventAdapterRuntimeInvocationSpecs.cs)
- Tests verify adapter runtime invocation with various configuration patterns
- All 20 Akka.Persistence.Hosting.Tests pass
- Verified backwards compatibility with existing EventAdapterSpecs

Related Issues:
- Fixes runtime invocation issue reported in akkadotnet/Akka.Persistence.Sql#552
- Critical fix for #665 callback API requirement

Author: Aaronontheweb