Refactor persistence manifest handling and add unit tests for loading behavior

[warwickschroeder]

An instance configured for SQL Server will not start. It dies with Could not load persistence customization type SQLServer. over a NullReferenceException.

No released version is affected.

What is wrong

PersistenceManifestLibrary reads every Persisters\<Name>\persistence.manifest in one loop wrapped by a single try, so the first file that throws discards every file after it, logged as a warning and nothing more.

The file that throws is RavenDB35. Its manifest has no AssemblyName or TypeName, correctly, because there is no assembly left to load, but the model marked both required and System.Text.Json rejects it. Folders are read in name order, so RavenDB35 throws third and SQLServer is never read. PostgreSQL sorts earlier and survives, which is what makes it look SQL-Server-specific.

It is invisible from a source checkout, where manifests come from DevelopmentPersistenceLocations and RavenDB 3.5 never appears. Only a real install shows it.

The fix

• Per-file try/catch in a new LoadManifests, so a bad manifest costs you that manifest and nothing else.
• AssemblyName and TypeName are nullable again. Name, DisplayName and Description keep required.

Tests

Two tests, one per defect, staging a real install layout in a temp directory. Each half of the fix was reverted separately to confirm the matching test goes red.