Remove support for #[sats(name = "SumNamespace.SumType")]

Custom #[sats(name = "SumNamespace.SumType")] syntax is allowed only for sum types and only in C# output. It was preserved for a long time for BitCraft backward compat. We don't support it anywhere else - in other languages the dot will be ignored and the type will be generated as just SumNamespaceSumType - and it's not part of our official API.

After talking with @aasoni, he confirmed BitCraft can easily migrate away from this syntax, so we should be able to remove it from SpacetimeDB as well, which should simplify a few places in the codebase. It will also close https://github.com/clockworklabs/SpacetimeDB/issues/1591 and https://github.com/clockworklabs/SpacetimeDBPrivate/issues/953. cc @kazimuth

In the new C# codegen I'm just assuming that we don't need this feature, and aligning the generated type with other languages (emitting a flattened SumNamespaceSumType), but not removing other places that handle such names. This issue is meant to track such hard-deprecation.

The issue identifies custom #[sats(name = "SumNamespace.SumType")] syntax, sum types, and C# codegen as the scope, but names no files or tests. Start by tracing existing handling of this attribute across output languages and compare the generated C# name with other languages. Done when the deprecated support is removed consistently and the linked compatibility issues can be closed.